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Chapter 13 Inter-Integrated Circuit (MC9S12XDP512) Block Description
MC9S12XDP512 Data Sheet, Rev. 2.11
Freescale Semiconductor 613
13.7.1.3 Post-Transfer Software Response
Transmission or reception of a byte will set the data transferring bit (TCF) to 1, which indicates one byte
communication is finished. The IIC bus interrupt bit (IBIF) is set also; an interrupt will be generated if the
interrupt function is enabled during initialization by setting the IBIE bit. Software must clear the IBIF bit
in the interrupt routine first. The TCF bit will be cleared by reading from the IIC bus data I/O register
(IBDR) in receive mode or writing to IBDR in transmit mode.
Software may service the IIC I/O in the main program by monitoring the IBIF bit if the interrupt function
is disabled. Note that polling should monitor the IBIF bit rather than the TCF bit because their operation
is different when arbitration is lost.
Note that when an interrupt occurs at the end of the address cycle the master will always be in transmit
mode, i.e. the address is transmitted. If master receive mode is required, indicated by R/W bit in IBDR,
then the Tx/Rx bit should be toggled at this stage.
During slave mode address cycles (IAAS=1), the SRW bit in the status register is read to determine the
direction of the subsequent transfer and the Tx/Rx bit is programmed accordingly. For slave mode data
cycles (IAAS=0) the SRW bit is not valid, the Tx/Rx bit in the control register should be read to determine
the direction of the current transfer.
The following is an example of a software response by a 'master transmitter' in the interrupt routine.
13.7.1.4 Generation of STOP
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.
If a master receiver wants to terminate a data transfer, it must inform the slave transmitter by not
acknowledging the last byte of data which 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.
ISR BCLR IBSR,#$02 ;CLEAR THE IBIF FLAG
BRCLR IBCR,#$20,SLAVE ;BRANCH IF IN SLAVE MODE
BRCLR IBCR,#$10,RECEIVE ;BRANCH IF IN RECEIVE MODE
BRSET IBSR,#$01,END ;IF NO ACK, END OF TRANSMISSION
TRANSMIT MOVB DATABUF,IBDR ;TRANSMIT NEXT BYTE OF DATA
MASTX TST TXCNT ;GET VALUE FROM THE TRANSMITING COUNTER
BEQ END ;END IF NO MORE DATA
BRSET IBSR,#$01,END ;END IF NO ACK
MOVB DATABUF,IBDR ;TRANSMIT NEXT BYTE OF DATA
DEC TXCNT ;DECREASE THE TXCNT
BRA EMASTX ;EXIT
END BCLR IBCR,#$20 ;GENERATE A STOP CONDITION
EMASTX RTI ;RETURN FROM INTERRUPT