Datasheet

Chapter 10 Inter-Integrated Circuit (IICV2)
MC9S12E128 Data Sheet, Rev. 1.07
Freescale Semiconductor 319
10.7.1.4 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.
10.7.1.5 Generation of Repeated START
At the end of data transfer, if the master continues to want to communicate on the bus, it can generate
another START signal followed by another slave address without first generating a STOP signal. A
program example is as shown.
10.7.1.6 Slave Mode
In the slave interrupt service routine, the module addressed as slave bit (IAAS) should be tested to check
if a calling of its own address has just been received. If IAAS is set, software should set the transmit/receive
mode select bit (Tx/Rx bit of IBCR) according to the R/W command bit (SRW). Writing to the IBCR
clears the IAAS automatically. Note that the only time IAAS is read as set is from the interrupt at the end
of the address cycle where an address match occurred, interrupts resulting from subsequent data transfers
will have IAAS cleared. A data transfer may now be initiated by writing information to IBDR, for slave
transmits, or dummy reading from IBDR, in slave receive mode. The slave will drive SCL low in-between
byte transfers, SCL is released when the IBDR is accessed in the required mode.
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
MASR DEC RXCNT ;DECREASE THE RXCNT
BEQ ENMASR ;LAST BYTE TO BE READ
MOVB RXCNT,D1 ;CHECK SECOND LAST BYTE
DEC D1 ;TO BE READ
BNE NXMAR ;NOT LAST OR SECOND LAST
LAMAR BSET IBCR,#$08 ;SECOND LAST, DISABLE ACK
;TRANSMITTING
BRA NXMAR
ENMASR BCLR IBCR,#$20 ;LAST ONE, GENERATE ‘STOP’ SIGNAL
NXMAR MOVB IBDR,RXBUF ;READ DATA AND STORE
RTI
RESTART BSET IBCR,#$04 ;ANOTHER START (RESTART)
MOVB CALLING,IBDR ;TRANSMIT THE CALLING ADDRESS;D0=R/W