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Embedded Systems Design: A Unified
Hardware/Software Introduction
Chapter 5 Memory
1
Introduction
• Embedded system’s functionality aspects
– Processing
• processors
• transformation of data
– Storage
• memory
• retention of data
– Communication
• buses
• transfer of data
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
2
Memory: basic concepts
• Stores large number of bits
…
m x n: m words of n bits each
k = Log2(m) address input signals
or m = 2^k words
e.g., 4,096 x 8 memory:
m words
–
–
–
–
m × n memory
…
n bits per word
• 32,768 bits
• 12 address input signals
• 8 input/output data signals
• Memory access
– r/w: selects read or write
– enable: read or write only when asserted
– multiport: multiple accesses to different locations
simultaneously
memory external view
r/w
2k × n read and write
memory
enable
A0
…
Ak-1
…
Qn-1
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
Q0
3
•
Traditional ROM/RAM distinctions
–
ROM
•
–
RAM
•
•
Advanced ROMs can be written to
•
–
Mask-programmed ROM
OTP ROM
EPROM
Tens of
years
Battery
life (10
years)
Ideal memory
EEPROM
FLASH
NVRAM
Nonvolatile
In-system
programmable
SRAM/DRAM
Near
zero
Write
ability
e.g., NVRAM
Write ability
–
Life of
product
e.g., EEPROM
Advanced RAMs can hold bits without
power
•
•
read and write, lose stored bits without
power
Traditional distinctions blurred
–
•
read only, bits stored without power
Storage
permanence
Write ability/ storage permanence
Manner and speed a memory can be
written
During
External
External
External
fabrication programmer, programmer, programmer
1,000s
OR in-system,
only
one time only
1,000s
of cycles
of cycles
External
In-system, fast
programmer
writes,
OR in-system,
unlimited
block-oriented
cycles
writes, 1,000s
of cycles
Storage permanence
–
ability of memory to hold stored bits
after they are written
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
Write ability and storage permanence of memories,
showing relative degrees along each axis (not to scale).
4
Write ability
•
Ranges of write ability
– High end
• processor writes to memory simply and quickly
• e.g., RAM
– Middle range
• processor writes to memory, but slower
• e.g., FLASH, EEPROM
– Lower range
• special equipment, “programmer”, must be used to write to memory
• e.g., EPROM, OTP ROM
– Low end
• bits stored only during fabrication
• e.g., Mask-programmed ROM
•
In-system programmable memory
– Can be written to by a processor in the embedded system using the
memory
– Memories in high end and middle range of write ability
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
5
Storage permanence
•
Range of storage permanence
– High end
• essentially never loses bits
• e.g., mask-programmed ROM
– Middle range
• holds bits days, months, or years after memory’s power source turned off
• e.g., NVRAM
– Lower range
• holds bits as long as power supplied to memory
• e.g., SRAM
– Low end
• begins to lose bits almost immediately after written
• e.g., DRAM
•
Nonvolatile memory
– Holds bits after power is no longer supplied
– High end and middle range of storage permanence
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
6
ROM: “Read-Only” Memory
– Store software program for general-purpose
processor
• program instructions can be one or more ROM
words
– Store constant data needed by system
– Implement combinational circuit
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
External view
2k × n ROM
enable
A0
…
• Nonvolatile memory
• Can be read from but not written to, by a
processor in an embedded system
• Traditionally written to, “programmed”,
before inserting to embedded system
• Uses
Ak-1
…
Qn-1
Q0
7
Example: 8 x 4 ROM
•
•
•
•
Horizontal lines = words
Vertical lines = data
Lines connected only at circles
Decoder sets word 2’s line to 1 if
address input is 010
• Data lines Q3 and Q1 are set to 1
because there is a “programmed”
connection with word 2’s line
• Word 2 is not connected with data
lines Q2 and Q0
• Output is 1010
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
Internal view
8 × 4 ROM
word 0
enable
3×8
decoder
word 1
word 2
A0
A1
A2
word line
data line
programmable
connection
wired-OR
Q3 Q2 Q1 Q0
8
Read-Only Memories
ROM: Two dimensional array of 1's and 0's
Row is called a "word"; index is called an "address"
Width of row is called bit-width or wordsize
Address is input, selected word is output
+5V +5V +5V +5V
n
2 -1
Dec
i
Word Line 0011
j
Word Line 1010
0
0
n-1
Address
Bit Lines
Internal Organization
Mask-programmed ROM
• Connections “programmed” at fabrication
– set of masks
• Lowest write ability
– only once
• Highest storage permanence
– bits never change unless damaged
• Typically used for final design of high-volume systems
– spread out NRE cost for a low unit cost
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
10
OTP ROM: One-time programmable ROM
• Connections “programmed” after manufacture by user
–
–
–
–
user provides file of desired contents of ROM
file input to machine called ROM programmer
each programmable connection is a fuse
ROM programmer blows fuses where connections should not exist
• Very low write ability
– typically written only once and requires ROM programmer device
• Very high storage permanence
– bits don’t change unless reconnected to programmer and more fuses
blown
• Commonly used in final products
– cheaper, harder to inadvertently modify
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
11
EPROM: Erasable programmable ROM
•
Programmable component is a MOS transistor
–
–
–
–
–
•
Transistor has “floating” gate surrounded by an insulator
(a) Negative charges form a channel between source and drain
storing a logic 1
(b) Large positive voltage at gate causes negative charges to
move out of channel and get trapped in floating gate storing a
logic 0
(c) (Erase) Shining UV rays on surface of floating-gate causes
negative charges to return to channel from floating gate restoring
the logic 1
(d) An EPROM package showing quartz window through which
UV light can pass
0V
floating gate
drain
source
(a)
+15V
(b)
source
drain
Better write ability
5-30 min
– can be erased and reprogrammed thousands of times
•
Reduced storage permanence
source
drain
(c)
– program lasts about 10 years but is susceptible to
radiation and electric noise
•
Typically used during design development
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
(d)
.
12
EEPROM: Electrically erasable
programmable ROM
• Programmed and erased electronically
– typically by using higher than normal voltage
– can program and erase individual words
• Better write ability
– can be in-system programmable with built-in circuit to provide higher
than normal voltage
• built-in memory controller commonly used to hide details from memory user
– writes very slow due to erasing and programming
• “busy” pin indicates to processor EEPROM still writing
– can be erased and programmed tens of thousands of times
• Similar storage permanence to EPROM (about 10 years)
• Far more convenient than EPROMs, but more expensive
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
13
Flash Memory
• Extension of EEPROM
– Same floating gate principle
– Same write ability and storage permanence
• Fast erase
– Large blocks of memory erased at once, rather than one word at a time
– Blocks typically several thousand bytes large
• Writes to single words may be slower
– Entire block must be read, word updated, then entire block written back
• Used with embedded systems storing large data items in
nonvolatile memory
– e.g., digital cameras, TV set-top boxes, cell phones
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
14
RAM: “Random-access” memory
• Typically volatile memory
– bits are not held without power supply
•
Read and written to easily by embedded system
during execution
• Internal structure more complex than ROM
external view
r/w
2k × n read and write
memory
enable
A0
…
Ak-1
…
Qn-1
– a word consists of several memory cells, each
storing 1 bit
internal view
I3 I2 I1 I0
– each input and output data line connects to each
cell in its column
– rd/wr connected to every cell
– when row is enabled by decoder, each cell has logic
that stores input data bit when rd/wr indicates write
or outputs stored bit when rd/wr indicates read
Q0
4×4 RAM
enable
2×4
decoder
A0
A1
Memory
cell
rd/wr
To every cell
Q3 Q2 Q1 Q0
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
15
Basic types of RAM
• SRAM: Static RAM
– Memory cell uses flip-flop to store bit
– Requires 6 transistors
– Holds data as long as power supplied
• DRAM: Dynamic RAM
memory cell internals
SRAM
Data'
Data
– Memory cell uses MOS transistor and
capacitor to store bit
– More compact than SRAM
– “Refresh” required due to capacitor leak
• word’s cells refreshed when read
W
DRAM
Data
W
– Typical refresh rate 15.625 microsec.
– Slower to access than SRAM
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
16
SRAM
DRAM
Random Access Memories
Dynamic RAMs
1 Transistor (+ capacitor) memory element
Word Line
Read: Assert Word Line, Sense Bit Line
Write: Drive Bit Line, Assert Word Line
Destructive Read-Out
Need for Refresh Cycles: storage decay in ms
Bit Line
Internal circuits read word and write back
Ram variations
• PSRAM: Pseudo-static RAM
– DRAM with built-in memory refresh controller
– Popular low-cost high-density alternative to SRAM
• NVRAM: Nonvolatile RAM
– Holds data after external power removed
– Battery-backed RAM
• SRAM with own permanently connected battery
• writes as fast as reads
• no limit on number of writes unlike nonvolatile ROM-based memory
– SRAM with EEPROM or flash
• stores complete RAM contents on EEPROM or flash before power turned off
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
19
A simple bus
• Wires:
– Uni-directional or bi-directional
– One line may represent multiple wires
• Bus
rd'/wr
Processor
– Set of wires with a single function
• Address bus, data bus
Memory
enable
addr[0-11]
data[0-7]
– Or, entire collection of wires
• Address, data and control
• Associated protocol: rules for
communication
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
bus
bus structure
20
Ports
Processor
port
rd'/wr
Memory
enable
addr[0-11]
data[0-7]
•
•
•
bus
Conducting device on periphery
Connects bus to processor or memory
Often referred to as a pin
– Actual pins on periphery of IC package that plug into socket on printed-circuit board
– Sometimes metallic balls instead of pins
– Today, metal “pads” connecting processors and memories within single IC
•
Single wire or set of wires with single function
– E.g., 12-wire address port
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
21
Timing Diagrams
•
•
•
Most common method for describing a
communication protocol
Time proceeds to the right on x-axis
Control signal: low or high
– May be active low (e.g., go’, /go, or go_L)
– Use terms assert (active) and deassert
– Asserting go’ means go=0
•
•
Data signal: not valid or valid
Protocol may have subprotocols
– Called bus cycle, e.g., read and write
– Each may be several clock cycles
•
Read example
– rd’/wr set low,address placed on addr for at
least tsetup time before enable asserted, enable
triggers memory to place data on data wires
by time tread
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
rd'/wr
enable
addr
data
tsetup
tread
read protocol
rd'/wr
enable
addr
data
tsetup
twrite
write protocol
22
RAM timing diagrams
Random Access Memories
RAM Timing
WE
CS
Simplified Read Timing
Address
Valid Address
Access Time
Data Out
Data Out
WE
CS
Simplified Write Timing
Memory Cycle T ime
Address
Valid Address
Data In
Input Data
Basic protocol concepts
•
•
•
Actor: master initiates, servant (slave) respond
Direction: sender, receiver
Addresses: special kind of data
– Specifies a location in memory, a peripheral, or a register within a peripheral
•
Time multiplexing
– Share a single set of wires for multiple pieces of data
– Saves wires at expense of time
Time-multiplexed data transfer
Master
req
data(15:0)
Servant
data(15:0)
mux
demux
Master
addr
data
Servant
addr
mux
data(8)
data
demux
addr/data
req
data
req
req
15:8
7:0
data serializing
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
addr/data
addr
data
address/data muxing
24
Contemporary Logic Design
Sequential Case Studies
Random Access Memories
DRAM Organization
Long rows to simplify refresh
Two new signals: RAS, CAS
Storage Matrix
Row
Decoders
64 x 64
Row Address Strobe
Column Address Strobe
replace Chip Select
Row Address
A11
. . .
A0
RAS
CAS
WE
Column Address &
Control Signals
Column Latches,
Multiplexers/Demultiplexers
Control
Logic
DOUT
DIN
ฉ R.H. Katz Transparency No. 7-1
Random Access Memory
RAS, CAS Addressing
Even to read 1 bit, an entire 64-bit row is read!
Separate addressing into two cycles: Row Address, Column Address
Saves on package pins, speeds RAM access for sequential bits!
Address
Row Address
Col Addres s
RAS
Read Cycle
CAS
Valid
Dout
Read Row
Row Address Latched
Read Bit Within Row
Column Address Latched
Tri-state
Outputs
Random Access Memory
Write Cycle Timing
Address
Row Address
Col Address
RAS
(1) Latch Row Address
Read Row
CAS
WE
(2) WE low
Din
(3) CAS low: replace data bit
(4) RAS high: write back the modified row
(5) CAS high to complete the memory cycle
Valid
Random Access Memory
RAM Refresh
Refresh Frequency:
4096 word RAM -- refresh each word once every 4 ms
Assume 120ns memory access cycle
This is one refresh cycle every 976 ns (1 in 8 DRAM accesses)!
But RAM is really organized into 64 rows
This is one refresh cycle every 62.5 ตs (1 in 500 DRAM accesses)
Large capacity DRAMs have 256 rows, refresh once every 16 ตs
RAS-only Refresh (RAS cycling, no CAS cycling)
External controller remembers last refreshed row
Some memory chips maintain refresh row pointer
CAS before RAS refresh: if CAS goes low before RAS, then refresh
Basic protocol concepts: control methods
Master
Servant
req
Master
Servant
req
ack
data
data
req
data
1
req
3
2
4
ack
1
3
2
4
data
taccess
1. Master asserts req to receive data
2. Servant puts data on bus within time taccess
3. Master receives data and deasserts req
4. Servant ready for next request
Strobe protocol
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
1. Master asserts req to receive data
2. Servant puts data on bus and asserts ack
3. Master receives data and deasserts req
4. Servant ready for next request
Handshake protocol
29
Memory Interface
No common clock between CPU and memory
Follow asynchronous 4-cycle handshake request/wait (ack) protocol
1. Request Asserted
Request
2. Wait Unasserted
Rea d/Write
3. Request Unasserted
Data
4. Wait Asserted
Wait
From Memory
Read Cycle
To Me mory
Write Cycle
Memory cannot make request unless Wait signal is asserted
Hi-to-Lo transition on Wait implies that data is ready (read)
or data has been latched by memory (write)
A strobe/handshake compromise
Master
req
Servant
wait
data
req
1
3
req 1
wait
data
wait
2
4
taccess
1. Master asserts req to receive data
2. Servant puts data on bus within time taccess
(wait line is unused)
3. Master receives data and deasserts req
4. Servant ready for next request
Fast-response case
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
4
2
3
data
5
taccess
1. Master asserts req to receive data
2. Servant can't put data within taccess, asserts wait ack
3. Servant puts data on bus and deasserts wait
4. Master receives data and deasserts req
5. Servant ready for next request
Slow-response case
31
Microprocessor interfacing: I/O addressing
• A microprocessor communicates with other devices
using some of its pins
– Port-based I/O (parallel I/O)
• Processor has one or more N-bit ports
• Processor’s software reads and writes a port just like a register
• E.g., P0 = 0xFF; v = P1.2; -- P0 and P1 are 8-bit ports
– Bus-based I/O
• Processor has address, data and control ports that form a single bus
• Communication protocol is built into the processor
• A single instruction carries out the read or write protocol on the bus
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
32
Compromises/extensions
• Parallel I/O peripheral
– When processor only supports bus-based I/O but
parallel I/O needed
– Each port on peripheral connected to a register
within peripheral that is read/written by the
processor
• Extended parallel I/O
– When processor supports port-based I/O but
more ports needed
– One or more processor ports interface with
parallel I/O peripheral extending total number of
ports available for I/O
– e.g., extending 4 ports to 6 ports in figure
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
Processor
Memory
System bus
Parallel I/O peripheral
Port A
Port B
Port C
Adding parallel I/O to a busbased I/O processor
Processor
Port 0
Port 1
Port 2
Port 3
Parallel I/O peripheral
Port A Port B Port C
Extended parallel I/O
33
Types of bus-based I/O:
memory-mapped I/O and standard I/O
• Processor talks to both memory and peripherals using
same bus – two ways to talk to peripherals
– Memory-mapped I/O
• Peripheral registers occupy addresses in same address space as memory
• e.g., Bus has 16-bit address
– lower 32K addresses may correspond to memory
– upper 32k addresses may correspond to peripherals
– Standard I/O (I/O-mapped I/O)
• Additional pin (M/IO) on bus indicates whether a memory or peripheral
access
• e.g., Bus has 16-bit address
– all 64K addresses correspond to memory when M/IO set to 0
– all 64K addresses correspond to peripherals when M/IO set to 1
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
34
Memory-mapped I/O vs. Standard I/O
• Memory-mapped I/O
– Requires no special instructions
• Assembly instructions involving memory like MOV and ADD work
with peripherals as well
• Standard I/O requires special instructions (e.g., IN, OUT) to move
data between peripheral registers and memory
• Standard I/O
– No loss of memory addresses to peripherals
– Simpler address decoding logic in peripherals possible
• When number of peripherals much smaller than address space then
high-order address bits can be ignored
– smaller and/or faster comparators
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
35
Microprocessor interfacing: interrupts
• Suppose a peripheral intermittently receives data,
which must be serviced by the processor
– The processor can poll the peripheral regularly to see if data
has arrived – wasteful
– The peripheral can interrupt the processor when it has data
• Requires an extra pin or pins: Int
– If Int is 1, processor suspends current program, jumps to an
Interrupt Service Routine, or ISR
– Known as interrupt-driven I/O
– Essentially, “polling” of the interrupt pin is built-into the
hardware, so no extra time!
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
36
Microprocessor interfacing: interrupts
• What is the address (interrupt address vector) of the
ISR?
– Fixed interrupt
• Address built into microprocessor, cannot be changed
• Either ISR stored at address or a jump to actual ISR stored if not
enough bytes available
– Vectored interrupt
• Peripheral must provide the address
• Common when microprocessor has multiple peripherals connected
by a system bus
– Compromise: interrupt address table
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
37
Interrupt-driven I/O using fixed ISR location
Time
1(a): μP is executing its main program.
3: After completing instruction at 100, μP
sees Int asserted, saves the PC’s value of
100, and sets PC to the ISR fixed location
of 16.
4(a): The ISR reads data from 0x8000,
modifies the data, and writes the resulting
data to 0x8001.
1(b): P1 receives input data in a
register with address 0x8000.
2: P1 asserts Int to request
servicing by the
microprocessor.
4(b): After being read, P1 deasserts Int.
5: The ISR returns, thus restoring PC to
100+1=101, where μP resumes executing.
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
38
Interrupt-driven I/O using fixed ISR location
1(a): P is executing its main program
1(b): P1 receives input data in a register
with address 0x8000.
Program memory
ISR
16: MOV R0, 0x8000
17: # modifies R0
18: MOV 0x8001, R0
19: RETI # ISR return
...
Main program
...
100: instruction
101: instruction
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
μP
Data memory
System bus
Int
PC
P1
P2
0x8000
0x8001
39
Interrupt-driven I/O using fixed ISR location
2: P1 asserts Int to request servicing by
the microprocessor
Program memory
ISR
16: MOV R0, 0x8000
17: # modifies R0
18: MOV 0x8001, R0
19: RETI # ISR return
...
Main program
...
100: instruction
101: instruction
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
μP
Data memory
System bus
Int
PC
P1
P2
0x8000
0x8001
1
40
Interrupt-driven I/O using fixed ISR location
3: After completing instruction at 100,
P sees Int asserted, saves the PC’s
value of 100, and sets PC to the ISR
fixed location of 16.
Program memory
ISR
16: MOV R0, 0x8000
17: # modifies R0
18: MOV 0x8001, R0
19: RETI # ISR return
...
Main program
...
100: instruction
101: instruction
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
μP
Data memory
System bus
Int
PC
P1
P2
0x8000
0x8001
100
41
Interrupt-driven I/O using fixed ISR location
4(a): The ISR reads data from 0x8000,
modifies the data, and writes the
resulting data to 0x8001.
4(b): After being read, P1 deasserts Int.
Program memory
ISR
16: MOV R0, 0x8000
17: # modifies R0
18: MOV 0x8001, R0
19: RETI # ISR return
...
Main program
...
100: instruction
101: instruction
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
μP
Data memory
System bus
Int
PC
P1
P2
0x8000
0x8001
0
100
42
Interrupt-driven I/O using fixed ISR location
5: The ISR returns, thus restoring PC to
100+1=101, where P resumes
executing.
Program memory
ISR
16: MOV R0, 0x8000
17: # modifies R0
18: MOV 0x8001, R0
19: RETI # ISR return
...
Main program
...
100: instruction
101: instruction
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
μP
Data memory
System bus
Int
PC
100
+1
P1
P2
0x8000
0x8001
43
Interrupt-driven I/O using vectored interrupt
Time
1(a): μP is executing its main program.
3: After completing instruction at 100, μP sees Int
asserted, saves the PC’s value of 100, and asserts
Inta.
5(a): μP jumps to the address on the bus (16).
The ISR there reads data from 0x8000, modifies
the data, and writes the resulting data to 0x8001.
1(b): P1 receives input data in a
register with address 0x8000.
2: P1 asserts Int to request servicing
by the microprocessor.
4: P1 detects Inta and puts interrupt
address vector 16 on the data bus.
5(b): After being read, P1 deasserts
Int.
6: The ISR returns, thus restoring PC to
100+1=101, where μP resumes executing.
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
44
Interrupt-driven I/O using vectored interrupt
1(a): P is executing its main program
1(b): P1 receives input data in a register
with address 0x8000.
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
Program memory
ISR
16: MOV R0, 0x8000
17: # modifies R0
18: MOV 0x8001, R0
19: RETI # ISR return
...
Main program
...
100: instruction
101: instruction
μP
Data memory
System bus
Inta
Int
PC
100
P1
P2
16
0x8000
0x8001
45
Interrupt-driven I/O using vectored interrupt
2: P1 asserts Int to request servicing by the
microprocessor
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
Program memory
ISR
16: MOV R0, 0x8000
17: # modifies R0
18: MOV 0x8001, R0
19: RETI # ISR return
...
Main program
...
100: instruction
101: instruction
μP
Data memory
System bus
Inta
Int
PC
100
P1
1
P2
16
0x8000
0x8001
46
Interrupt-driven I/O using vectored interrupt
3: After completing instruction at 100, μP
sees Int asserted, saves the PC’s value of
100, and asserts Inta
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
Program memory
ISR
16: MOV R0, 0x8000
17: # modifies R0
18: MOV 0x8001, R0
19: RETI # ISR return
...
Main program
...
100: instruction
101: instruction
μP
Data memory
System bus
Inta
Int
PC
100
1
P1
P2
16
0x8000
0x8001
47
Interrupt-driven I/O using vectored interrupt
4: P1 detects Inta and puts interrupt
address vector 16 on the data bus
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
Program memory
ISR
16: MOV R0, 0x8000
17: # modifies R0
18: MOV 0x8001, R0
19: RETI # ISR return
...
Main program
...
100: instruction
101: instruction
μP
Data memory
System bus
16
Inta
Int
PC
100
P1
P2
16
0x8000
0x8001
48
Interrupt-driven I/O using vectored interrupt
5(a): PC jumps to the address on the bus
(16). The ISR there reads data from
0x8000, modifies the data, and writes the
resulting data to 0x8001.
5(b): After being read, P1 deasserts Int.
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
Program memory
ISR
16: MOV R0, 0x8000
17: # modifies R0
18: MOV 0x8001, R0
19: RETI # ISR return
...
Main program
...
100: instruction
101: instruction
μP
Data memory
System bus
Inta
Int
PC
100
P1
0
P2
16
0x8000
0x8001
49
Interrupt-driven I/O using vectored interrupt
6: The ISR returns, thus restoring the PC to
100+1=101, where the μP resumes
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
Program memory
ISR
16: MOV R0, 0x8000
17: # modifies R0
18: MOV 0x8001, R0
19: RETI # ISR return
...
Main program
...
100: instruction
101: instruction
μP
Data memory
System bus
Int
PC
100
+1
P1
P2
0x8000
0x8001
50
Interrupt address table
• Compromise between fixed and vectored interrupts
– One interrupt pin
– Table in memory holding ISR addresses (maybe 256 words)
– Peripheral doesn’t provide ISR address, but rather index into
table
• Fewer bits are sent by the peripheral
• Can move ISR location without changing peripheral
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
51
Additional interrupt issues
• Maskable vs. non-maskable interrupts
– Maskable: programmer can set bit that causes processor to ignore
interrupt
• Important when in the middle of time-critical code
– Non-maskable: a separate interrupt pin that can’t be masked
• Typically reserved for drastic situations, like power failure requiring
immediate backup of data to non-volatile memory
• Jump to ISR
– Some microprocessors treat jump same as call of any subroutine
• Complete state saved (PC, registers) – may take hundreds of cycles
– Others only save partial state, like PC only
• Thus, ISR must not modify registers, or else must save them first
• Assembly-language programmer must be aware of which registers stored
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
52
Direct memory access
• Buffering
– Temporarily storing data in memory before processing
– Data accumulated in peripherals commonly buffered
• Microprocessor could handle this with ISR
– Storing and restoring microprocessor state inefficient
– Regular program must wait
• DMA controller more efficient
– Separate single-purpose processor
– Microprocessor relinquishes control of system bus to DMA controller
– Microprocessor can meanwhile execute its regular program
• No inefficient storing and restoring state due to ISR call
• Regular program need not wait unless it requires the system bus
– Harvard archictecture – processor can fetch and execute instructions as long as
they don’t access data memory – if they do, processor stalls
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
53
Peripheral to memory transfer without DMA,
using vectored interrupt
Time
1(a): μP is executing its main program.
3: After completing instruction at 100, μP sees Int
asserted, saves the PC’s value of 100, and asserts Inta.
1(b): P1 receives input data in a register
with address 0x8000.
2: P1 asserts Int to request servicing by
the microprocessor.
4: P1 detects Inta and puts interrupt
address vector 16 on the data bus.
5(a): μP jumps to the address on the bus (16). The ISR
there reads data from 0x8000 and then writes it to
0x0001, which is in memory.
5(b): After being read, P1 deasserts Int.
6: The ISR returns, thus restoring PC to 100+1=101,
where μP resumes executing.
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
54
Peripheral to memory transfer without DMA,
using vectored interrupt
1(a): P is executing its main program
1(b): P1 receives input data in a register
with address 0x8000.
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
Program memory
ISR
16: MOV R0, 0x8000
17: # modifies R0
18: MOV 0x0001, R0
19: RETI # ISR return
...
Main program
...
100: instruction
101: instruction
μP
Data memory
0x0000 0x0001
System bus
Inta
Int
PC
P1
16
0x8000
55
Peripheral to memory transfer without DMA,
using vectored interrupt
2: P1 asserts Int to request servicing by the
microprocessor
Program memory
ISR
16: MOV R0, 0x8000
17: # modifies R0
18: MOV 0x0001, R0
19: RETI # ISR return
...
Main program
...
100: instruction
101: instruction
μP
Data memory
0x0000 0x0001
System bus
Inta
Int
PC
P1
1
16
0x8000
100
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
56
Peripheral to memory transfer without DMA,
using vectored interrupt
3: After completing instruction at 100, P
sees Int asserted, saves the PC’s value of
100, and asserts Inta.
Program memory
ISR
16: MOV R0, 0x8000
17: # modifies R0
18: MOV 0x0001, R0
19: RETI # ISR return
...
Main program
...
100: instruction
101: instruction
μP
Data memory
0x0000 0x0001
System bus
1
Inta
Int
PC
P1
16
0x8000
100
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
57
Peripheral to memory transfer without DMA,
using vectored interrupt (cont’)
4: P1 detects Inta and puts interrupt address
vector 16 on the data bus.
Program memory
ISR
16: MOV R0, 0x8000
17: # modifies R0
18: MOV 0x0001, R0
19: RETI # ISR return
...
Main program
...
100: instruction
101: instruction
μP
Data memory
0x0000 0x0001
System bus
16
Inta
Int
PC
P1
16
0x8000
100
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
58
Peripheral to memory transfer without DMA,
using vectored interrupt (cont’)
5(a): P jumps to the address on the bus (16).
The ISR there reads data from 0x8000 and
then writes it to 0x0001, which is in memory.
5(b): After being read, P1 de-asserts Int.
Program memory
ISR
16: MOV R0, 0x8000
17: # modifies R0
18: MOV 0x0001,
0x8001, R0
19: RETI # ISR return
...
Main program
...
100: instruction
101: instruction
μP
Data memory
0x0000 0x0001
System bus
Inta
Int
PC
P1
0
16
0x8000
100
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
59
Peripheral to memory transfer without DMA,
using vectored interrupt (cont’)
6: The ISR returns, thus restoring PC to
100+1=101, where P resumes executing.
Program memory
ISR
16: MOV R0, 0x8000
17: # modifies R0
18: MOV 0x0001,
0x8001, R0
19: RETI # ISR return
...
Main program
...
100: instruction
101: instruction
μP
System bus
Inta
Int
PC
100
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
Data memory
0x0000 0x0001
P1
16
+1 0x8000
60
Peripheral to memory transfer with DMA
Time
1(a): μP is executing its main program.
It has already configured the DMA ctrl
registers.
4: After executing instruction 100, μP
sees Dreq asserted, releases the system
bus, asserts Dack, and resumes
execution. μP stalls only if it needs the
system bus to continue executing.
7(a): μP de-asserts Dack and resumes
control of the bus.
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
1(b): P1 receives input
data in a register with
address 0x8000.
3: DMA ctrl asserts Dreq
to request control of
system bus.
2: P1 asserts req to request
servicing by DMA ctrl.
5: (a) DMA ctrl asserts
ack (b) reads data from
0x8000 and (b) writes that
data to 0x0001.
6:. DMA de-asserts Dreq
and ack completing
handshake with P1.
7(b): P1 de-asserts req.
61
Peripheral to memory transfer with DMA
(cont’)
1(a): P is executing its main program. It has
already configured the DMA ctrl registers
1(b): P1 receives input data in a register with
address 0x8000.
Program memory
0x0000
Data memory
0x0001
No ISR needed!
System bus
...
Main program
...
100: instruction
101: instruction
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
μP
Dack
Dreq
PC
DMA ctrl
0x0001 ack
0x8000
req
P1
0x8000
100
62
Peripheral to memory transfer with DMA
(cont’)
2: P1 asserts req to request servicing
by DMA ctrl.
3: DMA ctrl asserts Dreq to request control of
system bus
Program memory
0x0000
Data memory
0x0001
No ISR needed!
System bus
...
Main program
...
100: instruction
101: instruction
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
μP
Dack
Dreq
1
PC
100
DMA ctrl
0x0001 ack
0x8000
P1
req
1
0x8000
63
Peripheral to memory transfer with DMA
(cont’)
4: After executing instruction 100, P sees
Dreq asserted, releases the system bus, asserts
Dack, and resumes execution, P stalls only if
it needs the system bus to continue executing.
Program memory
0x0000
Data memory
0x0001
No ISR needed!
System bus
...
Main program
...
100: instruction
101: instruction
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
μP
Dack
Dreq
PC
1
DMA ctrl
0x0001 ack
0x8000
req
P1
0x8000
100
64
Peripheral to memory transfer with DMA
(cont’)
5: DMA ctrl (a) asserts ack, (b) reads data
from 0x8000, and (c) writes that data to
0x0001.
Program memory
μP
0x0000
Data memory
0x0001
No ISR needed!
System bus
(Meanwhile, processor still executing if not
stalled!)
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
...
Main program
...
100: instruction
101: instruction
Dack
Dreq
PC
DMA ctrl
0x0001 ack
0x8000 req
1
P1
0x8000
100
65
Peripheral to memory transfer with DMA
(cont’)
6: DMA de-asserts Dreq and ack completing
the handshake with P1.
Program memory
μP
0x0000
Data memory
0x0001
No ISR needed!
System bus
...
Main program
...
100: instruction
101: instruction
Embedded Systems Design: A Unified
Hardware/Software Introduction, (c) 2000 Vahid/Givargis
Dack
Dreq
0
PC
DMA ctrl
0x0001 ack
0x8000
req
0
P1
0x8000
100
66