HC705E5GRSSLASHD datasheet(91/148 Pages) NXP

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Motorola Bus (M Bus) Interface

Programming Considerations

MC68HC705E5 — Rev. 1.0

General Release Specification

Motorola Bus (M Bus) Interface

11.8.4 Generation of the Stop Signal

A data transfer ends with a stop signal generated by the master device.

A master transmitter can simply generate a stop signal after all the data

has been transmitted.

The following is an example showing how a stop condition is generated

by a master transmitter.

MASTX

BRSET

0,MSR,END

; IF NO ACK, BRANCH TO END

LDA

TXCNT

; GET VALUE FROM THE

; TRANSMITTING COUNTER

BEQ

END

; IF NO MORE DATA, BRANCH TO

; END

LDA

DATABUF

; GET NEXT BYTE OF DATA

STA

MDR

; TRANSMIT THE DATA

DEC

TXCNT

; DECREASE THE TXCNT

BRA

EMASTX

; EXIT

END

BCLR

5,MCR

; GENERATE A STOP CONDITION

EMASTX

RTI

; RETURN FROM INTERRUPT

If a master receiver wants to terminate a data transfer, it must inform the

slave transmitter by not acknowledging the last byte of data. This can be

done by setting the transmit acknowledge bit (TXAK) before reading the

second to the last byte of data. Before reading the last byte of data, a

stop signal must be generated first.

The following is an example showing how a stop signal is generated by

a master receiver.

MASR

DEC

RXCNT

BEQ

ENMASR

; LAST BYTE TO BE READ

LDA

RXCNT

DECA

; CHECK LAST 2ND BYTE TO BE READ

BNE

NXMAR

; NOT LAST ONE OR LAST SECOND

LAMAR

BSET

3,MCR

; LAST SECOND, DISABLE ACK

; TRANSMITTING

BRA

NXMAR

ENMASR

BCLR

5,MCR

; LAST ONE, GENERATE 'STOP'

; SIGNAL

NXMAR

LDA

MDR

; READ DATA AND STORE

STA

RXBUF

RTI

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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