68HC705SB7 datasheet(91/170 Pages) FREESCALE

91 / 170 page

August 27, 1998

GENERAL RELEASE SPECIFICATION

MC68HC05SB7

SM-BUS

MOTOROLA

REV 2.1

12-13

ISR

BCLR

1,SMSR

; CLEAR THE SMIF FLAG

BRCLR

5,SMCR,SLAVE

; CHECK THE SMSTA FLAG,

; BRANCH IF SLAVE MODE

BRCLR

4,SMCR,RECEIVE

; CHECK THE MODE FLAG,

; BRANCH IF IN RECEIVE MODE

BRSET

0,SMSR,END

; CHECK ACK FROM RECEIVER

; IF NO ACK, END OF

; TRANSMISSION

TRANSMIT

LDA

DATABUF

; GET THE NEXT BYTE OF DATA

STA

SMDR

; TRANSMIT THE DATA

12.6.4 Generation of the STOP Signal

A data transfer ends with a STOP signal generated by the “master” device. A mas-

ter 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 gener-

ated by a master transmitter:

MASTX

BRSET

0,SMSR,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

SMDR

; TRANSMIT THE DATA

DEC

TXCNT

; DECREASE THE TXCNT

BRA

EMASTX

; EXIT

END

BCLR

5,SMCR

; 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 2nd 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,SMCR

; LAST SECOND, DISABLE ACK

; TRANSMITTING

BRA

NXMAR

ENMASR

BCLR

5,SMCR

; LAST ONE, GENERATE “STOP”

; SIGNAL

NXMAR

LDA

SMDR

; READ DATA AND STORE

STA

RXBUF

RTI

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