Download Chapter 8 Serial and Parallel Port Interfacing

Chapter 8 Serial and Parallel Port
Interfacing
Valvano’s Intro. To Embedded
Systems
8.1 General Intro. To Interfacing
• Three Components
– Mechanical Design of the Physical
Components (Selection of the parts.)
– Analog and Digital electronics used to
connect the physical devices.
– Low-Level Software (transforms the
mechanical and electrical devices into objects
that perform the desired tasks.
9S12 Interfacing
• 9s12 is built using CMOS
– Involves consideration of voltage, current, and
capacitance.
Digital Output
• IOH—largest current a port pin can source
when the output is high.
• VOH—smallest voltage a port pin can be if
the output is high.
• VDD—the power into the microcontroller
• Output High Voltage is between VDD and
VOH.
Digital Output (cont.)
• IOL—the largest current a port pin can
sink when the output is low.
• VOL—the largest voltage a port pin can be
if the output is low and the current is less
than IOL (the output low voltage will be
between 0 and VOL.
Digital Inputs
• IIH – the current the input will require
when the input is high.
• VIH—the voltage above which the input is
considered high.
• IIL—the current the input will require when
the input is low.
• VIL – voltage below which the input will be
considered low.
Capacitance
• Capacitance loading occurs with each
input and with long cables.
– Input— C=6pF
– Time constant CxR where R is the resistance
in the interface circuit.
– Example: V(t) = 5 – 5 exp(-t/RC)
• Output of one circuit is attached to the input of
another.
• If output goes from 0 to +5volts, the voltage is
perceived as V(t)
Device Driver
• A collection of software functions that
allow higher level software to utilize an I/O
device.
• Glossary definition( page 517): A
collection of software routines that perform
I/O functions.
8.2 Serial Communications
Interface, SCI
• Protocol is same as a UART.
• Old, but still in use on modern
microcontrollers.
• 8.2.1 discusses the RS232 Protocol.
• 8.2.2 discusses the transmission of an
asynchronous character.
• 8.2.3 discusses the reception of an
asynchronous character.
8.2.4 9S12 SCI Details
• The C32 has one SCI port and the DP512
has two ports.
• SCIDRL—SCI Data Register Low byte
– Transmit Data Register (Figure 8.3)
– Receive Data Register ( Figure 8.4)
– SCIBD—(SCI0BD,SCI1BD)—baud rate
register.
SCI Baud Rate
• SCI Baud Rate Control Register
– M—mode bit selects 8-bit (0) or 9-bit(1) data
frames.
– A frame always has a start bit and 1 or 2 stop
bits.
– Baud rate—total number of bits/second
– SCI Baud Rate= Mclk/16*BR
• BR is in bits 12:0 of the SCIBD
• Mclk is the module clock—typically the same as E
clock.
Table 8.4 (pg.290)
• SCICR2
–
–
–
–
–
–
–
–
TE(bit 3) --transmit enable.
RE(bit 2) –receive enable.
TIE(bit 7) –transmit interrupt enable.
TCIE(bit 6) – transmit complete interrupt enable.
RIE(bit 5)—receiver interrupt enable.
ILIE (bit 4)—idle line interrupt enable
RWU(bit 1)—recever wakeup control bit.
SBK(bit 0)– send break bit
Table 8.4 (cont.)
• SCICR1
– Loops (bit 7) –enables loop mode.
– RSRC (bit 5)—receiver source (when
loops=1)
– M (bit 4)—mode bit.
– WAKE (bit 3)-wake on idle line or msb
– ILT (bit 2) – idle line type (for the receiver)
– PE (bit 1)– parity enable.
– PT (bit 0)—parity type (0 for even parity)
Table 8.4
• SCISR1—SCI status register 1.
–
–
–
–
–
–
TDRE—(bit 7) –transmit data register empty.
TC—(bit 6)—transmit complete flag.
RDRF—(bit 5)—Receive Data Register Full
IDLE—(bit 4)—idle line detected
OR—(bit 3)—overrun error
NF—(bit 2) –noise flag (set if 3 samples are not the
same for a received bit.)
– FE—(bit 1)—frame error (set when no stop bit is
detected.)
– PF—(bit 0)—parity flag; (set when parity violation is
found and PE bit is set.)
Table 8.4
• SCISR2—SCI status register 2
– Contains two mode control bits and one
status bit.
• BRK13 (Bit 2)—Break Transmit Character Length
• TXDIR (Bit 1)—Transmitter Pin Data Direction
• RAF (BIT 0)—Receiver Active Flag
Busy-Waiting
• Same as gadfly or polling.
• Software continuously checks the
hardware status, waiting for it to be ready.
• Figure 8.6(page 293)—Flow Charts.
8.3 Serial (Synchronous)
Peripheral Interface
• Faster than SCI for interconnecting boards
and a PC.
8.3.1 SPI Fundamentals
• SPI used to attach additional I/O devices
(such as A/D’s and D/AC’s).
• SPI implements a synchronous protocol.
– Same clock is used.
8.3.1 SPI (cont.)
• Freescale SPI—4 I/O lines
– Slave Select --NOT–optional negative logic
control signal.
– SClk—50% duty cycle clock generated by the
master.
– MISO—master in, slave out data line.
– MOSI—master out, slave in data line.
SPI (cont.)
– Transmitting device uses one edge of the
clock for changing data, and the receiving
device uses the other edge.
• Controlled by CPOL(BIT 3) and CPHA(BIT 2) of
the SPICR1
• See Figures 8.7 and 8.8 on page 295.
• Pages 296-298 has more details.
8.3.4 8-bit DAC Interface
• Example 8.2 Design an 8-bit DAC with
range of 0 to +5.
8.4 Scanned Keyboards
• Switches are placed in a row/column
matrix.
• Fig. 8.12 (pg. 301) illustrates a matrix
keyboard.
• Computer drives one row at a time.
• Columns are read to determine which key
is pressed.
8.5 Parallel Port LCD Interface with
the HD44780 Controller
• The HD44780 Controller is an industry
standard for interfacing to a Liquid Crystal
Display.
• ASCII characters can be written to the
HD44780 controller.
• The ASCII character will be mapped onto
the display as a 5 by 8 bit pixel image.
8.6 Binary Actuators
• Relays, solenoids, and DC motors have
electrical interfaces that are similar.
8.7 Pulse-Width Modulation
• PWM can be used to deliver power in a
variable manner.
• The Duty Cycle can vary
– Duty = (H)/(H+L) (the fraction of the time that
the signal is high.
– Where H is the time that the signal is high.
– And L is the time that the signal is low.
– Figure 8.21 illustrates the wave forms.
8.8 Stepper Motors
• Evaluation of a motor:
– Maximum speed (RPM)
– Torque
– Efficiency of translating electrical power into
mechanical power
Stepper Motors (cont.)
• Stepper Motors—control the rotational
position.
• Used in applications where precise
positioning is more important than speed,
torque, or efficiency.