
MOTOROLA
SECTION 9: DDC12AB INTERFACE
Page 44
GENERAL RELEASE SPECIFICATION
MC68HC05BD7 Rev. 2.0
PRELIMINARY
TXBE
Bit 1
The Transmit Buffer Empty (TXBE) flag indicates the status of
the DDTR register. When the cpu writes the data into the
DDTR register, the TXBE flag will be cleared. And it will be set
again after the data of the DDTR register has been loaded to
the shift register. It is default to be set when the DEN is
disable and will be cleared by writing data to the DDTR
register when the DEN is enabled.
RXBF
Bit 0
The Receive Buffer Full (RXBF) flag indicates the status of the
DDRR register. When the cpu reads the data from the DDRR
register, the RXBF flag will be cleared. And it will be set after
the data or matched address is transferred from the shift
register to the DDRR register. It is cleared when DEN is
disabled or DDRR register is read when DEN is enabled.
9.3.5
DDC Data Transmit Register (DDTR)
The data written into this register after DEN is enabled will be automatically downloaded to
the shift register when the module detects the calling address is matched and the bit 0 of
the received data is one or when the data in the shift register has been transmitted with
received acknowledge bit, RXAK=0. So if the program doesn’t write the data into the DDTR
register (TXBE is cleared) before the matched calling address is detected, the module will
pull down the SCL line. If the cpu write a data to the DDTR register, then the written data
will be downloaded to the shift register immediately and the module will release the SCL
line, then the TXBE is set again and the TXIF flag is set to generate another interrupt
request for data. So the cpu may need to write the next data to the DDTR register to clear
TXBE flag and for the auto downloading of data to the shift register after the data in the shift
register is transmitted over again with RXAK=0. If the master receiver doesn’t acknowledge
the transmitted data, RXAK=1, the module will release the SDA line for master to generate
’stop’ or ’repeated start’ conditions. The data stored in the DDTR register will not be
downloaded to the shift register until next calling from master (TXBE remains unchanged).
9.3.6
DDC Data Receive Register (DDRR)
The DDC Data Receive Register (DDRR) contains the last received data when the MATCH
flag is zero or the calling address from master when the MATCH flag is one. The DDRR
register will be updated after a data byte is received and the RXBF is zero. It is a read-only
register. The read operation of this register will clear the RXBF flag. After the RXBF flag is
DTD7
1
7
DTD0
1111111
6543210
DTD4
W
R
DDTR
$001A
reset
DTD5
DTD6
DTD2
DTD3
DTD1
DRD7
0
7
DRD0
0000000
6543210
DRD4
W
R
DDRR
$001B
reset
DRD5
DRD6
DRD2
DRD3
DRD1