Starting with serial

Chapter Ten10.1, 10.2, 10.9-10.10.6

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Outline

• 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

16F873A16F876A

28 3 parallel ports,3 counter/timers,2 capture/compare/PWM,2 serial,5 10-bit ADC,2 comparators

16F874A16F877A

40 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)

• 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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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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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’ ;port is on, 8-bit,

movwf rcsta ;continuous receiving

bsf status,rp0

movlw B’00100100’ ;TX enabled, 8-bit,

movwf txsta ;high speed baud rate

movlw 04 ;baud rate = 50k @4MHz

movwf spbrg

bcf status,rp0

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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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