Starting with serial
Chapter Ten
10.1, 10.2, 10.9-10.10.6
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Outline
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Introduction
Synchronous data communication
Asynchronous data communication
The 16F87XA USART
Summary
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Introduction
Data transfer methods:
1. Parallel Transfer
– Faster
– Expensive
– Short distances
2. Serial Transfer
– Slower
– Cheaper
– Short and long distances
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Serial Data Transfer
Synchronous
Asynchronous
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The PIC 16 Series
Device
Pins
Features
16F873A
16F876A
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3 parallel ports,
3 counter/timers,
2 capture/compare/PWM,
2 serial,
5 10-bit ADC,
2 comparators
16F874A
16F877A
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5 parallel ports,
3 counter/timers,
2 capture/compare/PWM,
2 serial,
8 10-bit ADC,
2 comparators
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PIC 16F86XA Serial Ports
1. MSSP: The Master Synchronous Serial Port is
designed to support:
– SPI: Serial Peripheral Interface (Motorola )
– I2C: Inter-Integrated Circuit (Philips)
2. USART: Universal Synchronous Asynchronous
Receiver Transmitter can operate in both
synchronous and asynchronous modes.
– RS-232
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Synchronous data communication
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Shift Register to Receive Data
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Synchronous Signals
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A general-purpose serial
communication link
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Master/Slave Implementation
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Single synchronous master with
multiple slaves
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Disadvantages of synchronous
communication
• An extra line is needed to go to every data
node for the clock
• The bandwidth needed for the clock is always
twice the bandwidth needed for the data
• Over long distances, clock and data
themselves could lose synchronization
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Asynchronous principles
• No clock transmitted
• Data rate is predetermined – both transmitter
and receiver are preset to recognize the same
data rate.
• Each node needs an accurate and stable clock
source.
• Each byte or word is framed with a Start and Stop
bit. These allow synchronization to be initiated
before the data starts to flow.
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A common asynchronous serial
data format
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Synchronizing the asynchronous
data signal
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The 16F87XA USART
• USART: Addressable Universal Synchronous
Asynchronous Receiver Transmitter
• Modes:
– Synchronous master
– Synchronous slave
– Asynchronous full-duplex
• Has receive and transmit interrupts
• Controlled by TXSTA, RCSTA, and SPBRG
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TXSTA: transmit status and control
register (address 98h)
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CSRC:
TX9:
TXEN:
SYNC:
U:
BRGH:
TRMT:
TX9D:
clock source select
9-bit transmit enable
transmit enable
USART mode select
unimplemented
high baud rate select
transmit shift register status
9th bit of transmit data
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RCSTA: receive status and control
register (address 18h)
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SPEN:
RX9:
SREN:
CREN:
ADDEN:
FERR:
OERR:
RX9D:
serial port enable
9-bit receive enable
single receive enable
continuous receive enable
address detect enable
framing error
overrun error
9th bit of received data
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SPBRG: baud rate generator
(address 99h)
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USART transmit block diagram
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USART receive block diagram
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Serial Communications Example
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Asynchronous Data Transfer
Example – Page 1
;Initialise USART
bcf
status,rp0
movlw B’10010000’
movwf rcsta
bsf
status,rp0
movlw B’00100100’
movwf txsta
movlw 04
movwf spbrg
bcf status,rp0
...
;port is on, 8-bit,
;continuous receiving
;TX enabled, 8-bit,
;high speed baud rate
;baud rate = 50k @4MHz
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Asynchronous Data Transfer
Example – Page 2
;*********************************************
;ISR. On external interrupt, SSP reads byte
;from Hand Controller, sends it out on USART,
;receives it back through USART,
;and echoes it back to keypad.
;Received Byte stored in I2C_RX_word
;*********************************************
Interrupt_SR
...
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Asynchronous Data Transfer
Example – Page 3
;send out via async comm channel
bcf
pir1,rcif ;clear RX interrupt flag
movf I2C_RX_word,0 ;get word
movwf txreg
btfss pir1,rcif ;test for RX INT flag,
;indicating receive complete
goto $-1
movf rcreg,0 ;get and store RX word
movwf async_RX_word
...
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Asynchronous Waveform
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Using address detection with the
USART receive mode
• Multiple nodes can be connected to the serial line
and a node can recognize its own address.
• Set 9-bit mode RX9 and address enable bit ADDEN
• Logic 1 in the ninth bit indicates that an address is
received.
• If byte equals own address, revert to normal
reception by resetting ADDEN
• This continues until a further address word is
detected, which may be for another node.
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Summary
• There are two broad types of serial communication:
synchronous and asynchronous.
• There are a very large number of different standards and
protocols for serial communication, ranging from the very
simple to the seriously complicated. It is important to
match the right protocol with the right application.
• The 16F873A microcontroller has two extremely flexible
serial ports. The cost of flexibility is a significant level of
complexity in grasping their use. Therefore, it is often
worth adapting publicly available routines to use, rather
than starting from scratch in writing new code.
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