1. Starting with serial Chapter Ten 10.1, 10.2, 10.9-10.10.6
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2. Outline Introduction Synchronous data communication
Asynchronous data communication
The 16F87XA USART
Summary
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3. Introduction Data transfer methods: Parallel Transfer Serial Transfer
Faster
Expensive
Short distances
Serial Transfer
Slower
Cheaper
Short and long distances
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4. Serial Data Transfer
Synchronous
Asynchronous
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5. The PIC 16 Series Device Pins Features 16F873A 16F876A 28
3 parallel ports,
3 counter/timers,
2 capture/compare/PWM,
2 serial,
5 10-bit ADC,
2 comparators
16F874A
16F877A 40
5 parallel ports,
8 10-bit ADC,
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6. PIC 16F86XA Serial Ports
MSSP: The Master Synchronous Serial Port is designed to support:
SPI: Serial Peripheral Interface (Motorola )
I2C: Inter-Integrated Circuit (Philips)
USART: Universal Synchronous Asynchronous Receiver Transmitter can operate in both synchronous and asynchronous modes.
RS-232
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7. Synchronous data communication
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8. Shift Register to Receive Data
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9. Synchronous Signals
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10. A general-purpose serial communication link
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11. Master/Slave Implementation
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12. Single synchronous master with multiple slaves
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13. 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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14. 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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15. A common asynchronous serial data format
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16. Synchronizing the asynchronous data signal
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17. 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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18. TXSTA: transmit status and control register (address 98h)
CSRC: clock source select
TX9: 9-bit transmit enable
TXEN: transmit enable
SYNC: USART mode select
U: unimplemented
BRGH: high baud rate select
TRMT: transmit shift register status
TX9D: 9th bit of transmit data
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19. RCSTA: receive status and control register (address 18h)
SPEN: serial port enable
RX9: 9-bit receive enable
SREN: single receive enable
CREN: continuous receive enable
ADDEN: address detect enable
FERR: framing error
OERR: overrun error
RX9D: 9th bit of received data
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20. SPBRG: baud rate generator (address 99h)
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21. USART transmit block diagram
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22. USART receive block diagram
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23. Serial Communications Example
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24. Asynchronous Data Transfer Example – Page 1
;Initialise USART bcf status,rp0 movlw B’ ’ ;port is on, 8-bit, movwf rcsta ;continuous receiving bsf status,rp0 movlw B’ ’ ;TX enabled, 8-bit, movwf txsta ;high speed baud rate movlw 04 ;baud rate = movwf spbrg bcf status,rp0 ...
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25. 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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26. 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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27. Asynchronous Waveform
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28. 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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29. 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.
Dr. Gheith Abandah