Download Lecture-12-Communication Peripherals

The University of Texas at Arlington
Lecture 12
Communication Peripherals
CSE 3442/5442
PIC Communication
Peripherals
•  PIC18 family microcontrollers can have several built in
peripherals (modules) for serial communication:
–  UART/USART/EUSART for asynchronous or
synchronous communication, e.g., with computers
through COM serial ports
–  SPI for 3-wire or 4-wire synchronous communication
with peripherals such as external memory or other
peripherals
–  I2C port for communication with other I2C peripherals (3
wires).
•  A specific microcontroller can have more than one such
module (e.g., two USART modules)
•  Some modules can serve several communication modes
(e.g., EUSART, or MSSP – could be SPI or I2C)
2
Serial Communication
•  Digital devices that need to communicate can do that
through serial or parallel communication
•  Serial communication requires less wires (and less pins),
and due to crosstalk on long cables, can reach higher
total bandwidths.
•  Serial communication requires as little as two wires: one
for data and one for common ground.
•  In order to enable full-duplex communication, we can
introduce one more data line, and thus have a unique
data line in both directions.
•  If clock is not transmitted on a separate wire, then we are
talking about asynchronous serial communication
3
Asynchronous Serial Communication
•  Most desktops and early laptops used to have so called COM
asynchronous serial communication ports (before USB took over
entirely).
•  Asynchronous serial communication was very important from the
beginning for computers to talk to peripherals and thus good
standards have been established early (e.g., RS232C-1969).
•  Generally, there are three wires needed for RS232-like
communication: transmit (TxD), receive (RxD), and common
(ground).
•  For this type of communication to work, both ends need to be
running clocks at the same rate.
•  The rate at which bits are transmitted is measured in bps or (as one
symbol is transmitted in one clock step) baud.
•  Common (standard) baud rates are: 1.2k, 2.4k, 4.8k, 9.6k, 19.2k,
38.4k, 57.6k, 115.2k (but could be any other)
4
Async. Serial Communication –
Data Framing
•  Several bits are transmitted in the same word, but most usually 7,8,
or 9.
•  The data line has a default high level.
•  Transmissions start with one clock cycle worth of 0, a.k.a., start bit
•  Then the 7,8, or 9 bits of data follow
•  Then an optional parity bit is added (1 if the total number of 1-s in
the data are odd).
•  Finally, one, or two stop bits follow (stop bits have a value of 1).
•  It is of extreme importance that both sides are preset not only with
the baud rate, but the number of bits, the number of stop bits and
whether to use a parity bit. Otherwise they would miscommunicate.
Overhead?
time
LSB
5
RS232 Voltage Levels
•  What voltage level represents 1’s and 0’s?
–  0V, and 5V (TTL levels) were not well established at the creation of this
standard, plus they would result in DC level being present on the line
–  Logical 1 is represented by a negative voltage between -3V and -25V
(-15V being common)
–  Logical 0 is represented by a positive voltage between 3V and 25V (15V
being common)
•  As microcontrollers usually output 0V for “0” and 1-5V for “1”,
external voltage level converters are needed. This requires up
conversion. MAX232 is the most common conversion circuit.
6
How About Those Other RS232 Pins?
•  In addition to RxD, TxD and ground, there
are many other signals on RS232
connectors (that are not usually used on
microcontrollers)
•  They include: shielding, !RTS and !CTS for
flow control, DTR and DSR to indicate to
the other side that the terminal or the
modem is ready, DCD so the modem can
tell the computer if a line is available, etc.
7
PIC18 EUSART
•  The enhanced universal synchronous,
asynchronous receiver transmitter is one
peripheral module on the PIC18 that can be
used for serial communication.
•  Can be configured as:
–  ART full duplex
–  SRT, master
–  SRT, slave
•  Usually associated with PORTC, TRISC has to
be thus set appropriately.
8
SFRs
•  Six SFRs:
–  SPBGR (serial port baud rate generator)
–  TXREG (transfer register)
(copied to TSR(terminal shift register. Not accessible to the
programmer)
–  RCREG (receive register)
–  TXSTA (transmit status and control register)
–  RCSTA (receive status and control register)
–  PIR1 (peripheral interrupt request register1)
9
Baud Rate Generator
•  SFR registers are used in the PIC18 to set up the correct
baud rate. The Baud rate generator uses these registers.
•  BRG is either 8-bit, or 16-bit (changed by
BRG16=BAUDCON.3). (BAUDCON is a EUSART feature)
•  Depending on BRG16, and BRGH (TXSTA.2), the baud
rate is set in different ways. Registers SPBRGH and
SPBRG are to set the baud rate (by the user).
•  Note, that the baud rate needs to be calculated to be as
close to the nominal rate as possible.
10
Error in the Baud Rate
•  Rarely will the formula provided on the
previous page result in an exact baud rate.
•  The relative error in the set baud rate can
be easily calculated by:
relative error[%] = 100*(calculated rate – nominal rate)/ nominal rate
11
Error in the Baud Rate (cont’d)
•  The data on
the Rx pin is
sampled three
times (each
bit) and a
majority select
circuit decides
on its value.
12
USART Reception Control
Register: RCSTA
13
Receiving on UART
• 
• 
• 
• 
• 
• 
• 
RX pin needs to be set as input
Baud rate has to be set.
USART peripheral needs to be enabled (RCSTA.SPEN)
Receiving on UART needs to be enabled (RCSTA.CREN)
The PIR1.RCIF flag will be raised by the microcontroller if it received a valid byte.
The user needs to copy the received byte out of RCREG. This will automatically reset
RCIF.
Then the microcontroller can receive the next byte without overwriting RCREG (and
thus the user loosing data).
14
USART Transmission Control
Register: TXSTA
15
Transmitting on UART
• 
• 
• 
• 
TXREG is an 8-bit register that the user needs to load up with the data to be
transmitted. TSR is a shift register that is not user accessible.
The program needs to load a byte to be transmitted into TXREG. If TSR is
empty this same byte will be loaded into TSR and the TXREG is cleared (to
avoid sending the same byte twice). The program can then load another
value into the TXREG.
After loading the TXREG, the program needs to tell the microcontroller that
transmission should start by flipping the TXEN bit in the TXSTA control
register.
The TXIF (interrupt flag) in PIR1 is set by the microcontroller to notify the
program that the last byte has been sent. The program can then load the
next byte into TXREG. TXIF cannot be reset from software, it is cleared one
instruction cycle after loading the TXREG.
16
Transmitting Code Sample
#include <P18F458.h>
void main(void)
{
TXSTA = 0x20;
SPBRG = 15; //choose 9.6k baud at 10MHz xtal
TXSTAbits.TXEN = 1;
//enable transmission
RCSTAbits.SPEN = 1;
//turn on peripheral
while (1)
{
TXREG=‘Z’; //byte to be transmitted
while(PIR1bits.TXIF == 0); //wait until transmitted
}
}
17
UART Reception Sample Code
#include <P18F458.h>
void main(void)
{
TRISB = 0;
TRISC = 128; //setting RX bit as input
RCSTA = 0x90; //enable serial port and receiver
SPBRG = 15; //choose 9.6k baud at 10MHz xtal
while (1)
{
while(PIR1bits.RCIF == 0); //wait to receive
PORTB=RCREG;
//byte received copied to PORTB
}
}
18
EUSART Synchronous Mode
•  Brief description (used rarely)
•  Transmission/reception is half-duplex, master
needs to set communication direction
•  Microcontroller needs to determine if it is a
master (generating clock) or a slave (receiving
clock)
•  Thus scheduling of data transmission between
master and slave needs to be handled.
•  RX and TX pins become DT (data) and CK
(clock) respectively.
19
MSSP Peripheral Module
•  PIC18 microcontrollers can have a Master
Synchronous Serial Port (MSSP) peripheral. The
USART was good for point-to-point connections,
the MSSP is good to establish multi-access
buses.
•  The MSSP can operate in two modes:
–  SPI: Serial Peripheral Interface (we have seen this
when talking to the DAC)
–  I2C: Inter-Integrated Circuit, a popular two-wire,
addressable connection bus (proprietary)
•  We are briefly going to cover these two bus
technologies without going into detail how they
can be used in the PIC18.
20
SPI
•  SPI can be either 3-wire or 4-wire (they are not
compatible).
•  SDI (serial data in), SDO (serial data out), SCLK,
and CE (chip enable) make up the 4 signals. 3wire has SDI and SDO connected.
•  We have seen the SPI
module already with the
DAC.
•  SPI frame format contains
addresses in addition to
data
21
Writing to SPI Devices
•  Microcontroller assumes the role of a Master and
thus generates the clock.
•  CE needs to be set high indicating a transaction
on SPI. A high bit is clocked out on SDI,
indicating a write transaction, followed by a 7-bit
address. Then the data that needs to go to that
address is clocked out (by multiples of 8 bits,
thus address is auto incremented).
22
Reading from SPI
•  Microcontroller assumes the role of a Master and thus
generates the clock for the entire transaction.
•  CE needs to be set high indicating a transaction on SPI.
A low bit is clocked out by the master on SDI, indicating
a read transaction, followed by a 7-bit address. The
slave then will respond (clock out) the data stored at that
address (by multiples of 8 bits, thus address is auto
incremented).
23
I2C
•  Common bus speeds are 10kbps and 100kbps
but can be arbitrary.
•  Master node generates clock; four potential
modes: master transmits, master receives, slave
transmits, slave receives.
•  Control to the lines is usually open-collector
(open-drain).
•  Seven, or ten bits are used for addressing
peripheral, then data is transmitted.
24
Summary
•  Communicating to external peripherals, other
microcontrollers, or a computer are must in
embedded systems.
•  Microcontrollers usually provide several different
serial communication methods for the above.
•  UART is probably the most common technique
used in point-to-point communication, however
synchronous communication may be used as
well.
•  SPI and I2C are two bus standards that allow
several microcontrollers or peripheral devices to
be connected to the same bus.
25