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Serial I/O
Programming Unit, Lecture 5
LSU 10/22/2004
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Serial I/O with the BASIC Stamp
•The digital I/O examples have read or written a single bit of
information, a switch being open or closed, an LED being on or off.
Many applications require transferring groups of bits, bytes or words.
•Several of the digital I/O pins could be used in parallel for multi-bit I/O
but this has the disadvantage of quickly running out of pins or increasing
the complexity of the hardware with many wires or circuit board traces.
•An efficient alternative is to send many bits serially in time over a
single wire or connection. This simplifies the hardware but does require
software to manage the serial data stream.
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Parallel Interface
•Data lines may be unidirectional or bi-directional.
•Width of data bus is usually byte-wide (8 data bits).
•A full byte of data is transferred on each R/W clock cycle.
•Chip Select (CS) allows multiple devices to share bus.
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Serial Interface
•Asynchronous serial can be implemented with data lines only.
–Each device generates its own clock (Baud Rate Generator).
–Handshaking lines can be used to signal status of devices.
•Synchronous serial interfaces will have a separate Clock line.
–Clock is generated by a Master device.
•One bit is transferred for each clock cycle.
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Synchronous and Asynchronous Serial
Communication
•The BASIC Stamp communicates with the host PC through an
asynchronous serial port, usually referred to as a serial port,
comm port, or RS-232 port. The RS-232 standard specifies the
communications protocol, voltage levels and signal wiring so
that RS-232 devices can communicate with each other, at least
most of the time. Modems, printers, video terminals and other
devices frequently have asynchronous serial ports.
•Synchronous serial I/O is often used to interface specialized
integrated circuits to the BASIC Stamp. Synchronous I/O uses
a separate clock signal generated by the Master device (BASIC
Stamp).
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Asynchronous Serial Communication
The RS-232 standard specifies logic levels that can
typically exceed +10 or -10 volts. These levels can
destroy a BASIC Stamp. Appropriate level conversion
must be used to interface RS-232 devices to the I/O pins
of the Stamp. USE CAUTION!
The BASIC Stamp has a dedicated debug port which can
safely connect to RS-232 but it is a simplified interface
which may not work with all RS-232 devices.
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Asynchronous Serial Communication
Serial communication usually involves sending or receiving
“characters” using the ASCII code. For example the character “S” is
represented by the binary number “01010011” or 53 in hexadecimal.
An asynch transmission of “S” begins with a start bit, followed by 8
data bits and ending with a stop bit. There are numerous options for
number data bits, speed and an optional parity bit.
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Using the SEROUT Command
•The SEROUT command is used to send serial data. Any of the I/O pins P0
through P15 can be used for serial data. In addition the BASIC Stamp BS2P
has the dedicated DEBUG port for downloading and debugging programs
on the host PC. The Debug port can be referred to as logical port P16 and
corresponds to physical pin 1, called SOUT.
•Syntax for the simplest use of SEROUT:
SEROUT pin, Baudmode, [Outputdata]
For example SEROUT P4, 16468, [“Testing”]
This would send the stream of ASCII characters Testing out of I/O logical
pin 4. See the PBASIC Help on SEROUT command for details. The above
command sets the communication speed to 9600 baud (~bits per second).
The DEBUG command automatically uses P16 at 9600 baud.
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Using the SERIN Command
•Like the SEROUT command, the SERIN command is for asynchronous
serial I/O but in this case is for receiving data by the BASIC Stamp. The I/O
pins can be set to P0 through P15 or to the Debug port.
•The simplest usage of SERIN is
SERIN pin, Baudmode, [InputData]
SERIN has powerful optional features for waiting on input data, filtering
and formatting the received data. See the PBASIC Help on SERIN for
details.
One possible application using SERIN would be receiving data from a
Global Positioning System (GPS) module which typically defaults to
asynchronous data at 4800 baud.
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Synchronous Serial I/O
Synchronous serial I/O uses a separate line for a CLOCK signal. The
voltage levels used for the synchronous serial clock and data lines are
the TTL logic levels that are native to the BASIC Stamp so no level
converters or line drivers/receivers are required.
There are several protocols in use. Some use a bi-directional data line
while others use separate Data-In and Data-Out lines. The Master
generates the clock and initiates and controls data transfer.
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The I2C Bus
•Inter-Integrated-Circuit or I2C (pronounced I-too-see or I-squared-see)
is a synchronous serial protocol that uses a bi-directional data line and
supports multiple slave devices controlled by a I2C bus master.
•Defined by Phillips Semiconductor and became an industry standard.
•The clock line is called SCL, the data line SDA
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•The I2C bus master generates SCL and initiates communication with
one of the slave devices. Each device has a unique address for device
selection.
•A device address can be a combination of bits that are “hard-wired”
into the chip design and one or more pins on the device. These pins can
be wired High or Low to select an address that doesn’t conflict with
other devices on the I2C bus.
•Pull-up resistors are required on both the clock and data lines.
•A variety of special function integrated circuits are available with I2C
interfaces, including memory chips, analog-to-digital converters,
digital-to-analog converters and real-time-clocks.
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I2C START and STOP
•A START sequence begins a bus transmission by transitioning SDA
from High to Low while SCL is High.
•A STOP sequence ends a transmission. The Stop sequence occurs
when the master brings SDA from Low to High while SCL is High.
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I2C Write Sequence
•A typical I2C bus sequence for writing to a slave:
•Send a START sequence
•Send the I2C device address with the R/W Low (for Write)
•Send the data byte
•Optionally send additional data bytes (after repeating START)
•Send the STOP sequence after all data bytes have been sent
•The Slave responds by setting the ACK bit (Acknowledge)
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I2C Read Sequence
•Reading an I2C Slave device usually begins by writing to it. You must
tell the chip which internal register you want to read.
•I2C Read Sequence
•Send the START condition
•Send the device address with R/W held Low (for a Write)
•Send the number of the register you want to read
•Send a repeated START condition
•Send the device address with R/W set High (for a Read)
•Read the data byte from the slave
•Send the STOP sequence
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I2C Read example using device address 1100000 and
reading register number 1.
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I2C Programming on the BASIC Stamp
The BASIC Stamp can be programmed to operate as an I2C bus
master by “brute force” using the HIGH and LOW instructions
but PBASIC provides several instructions designed to make
synchronous serial and I2C communications less tedious.
The I2CIN and I2COUT instructions do most of the low level
dirty work of properly sequencing the SDA and SCL signals, but
only a subset of the I/O pins can be used for the I2C bus physical
layer.
If I/O pin P0 is selected to be SDA, then SCL is I/O pin P1.
If I/O pin P8 is selected to be SDA, then SCL is I/O pin P9.
The I2C instructions work with only these two pin configurations.
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The I2COUT Instruction
Syntax: I2COUT pin, slaveID, address, OutputData
pin specifies SDA and can only be 0 or 8. SCL will then be pin 1 or 9.
slaveID is the slave device address
address is the register address within the slave device
OutputData is the data to write to the I2C slave device
The I2COUT instruction automatically sets the R/W bit.
The I2COUT instruction supports multiple address bytes which may be
required for some slave devices. See the PBASIC Help for details.
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The I2CIN Instruction
Syntax: I2CIN pin, slaveID, address, InputData
pin specifies SDA and can only be 0 or 8. SCL will then be pin 1 or 9.
slaveID is the slave device address
address is the register address within the slave device
InputData is the data read from the I2C slave device
The I2IN instruction automatically sets the R/W bit.
The I2IN instruction supports multiple address bytes which may be
required for some slave devices. See the PBASIC Help for details.
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Other Synchronous Serial Protocols
Several IC manufacturers have developed their own protocols for
synchronous serial communication. The data sheets and application
notes from the chip manufacturer should be consulted for design and
troubleshooting tips.
•A few popular serial I/O protocols
–MICROWIRE (National Semiconductor)
–SPI (serial peripheral interface from Motorola)
–OneWire (Dallas Semiconductor)
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