Справочник Пользователя для Freescale Semiconductor Demonstration Board for Freescale MC9S12XHY256 Microcontroller DEMO9S12XHY256 DEMO9S12XHY256

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Freescale Semiconductor

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.

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.

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.

In slave transmitter routine, the received acknowledge bit (RXAK) must be tested before transmitting the
next byte of data. Setting RXAK means an 'end of data' signal from the master receiver, after which it must
be switched from transmitter mode to receiver mode by software. A dummy read then releases the SCL
line so that the master can generate a STOP signal.

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