MC68HC908AS32A Datasheet, PDF (91/280 Pages) Freescale Semiconductor, Inc – Microcontrollers

BDLC CPU Interface
TMIFR1 — Transmit Multiple Byte IFR with CRC (Type 3) Bit
The TMIFR1 bit requests the BDLC to transmit the byte in the BDLC data register (BDR) as the first
byte of a multiple byte IFR with CRC or as a single byte IFR with CRC. Response IFR bytes are still
subject to J1850 message length maximums. See 4.4.2 J1850 Frame Format and Figure 4-19.
If this bit is set prior to a valid EOD being received with no CRC error, once the EOD symbol has been
received the BDLC will attempt to transmit the appropriate normalization bit followed by IFR bytes. The
programmer should set TEOD after the last IFR byte has been written into the BDR register. After
TEOD has been set and the last IFR byte has been transmitted, the CRC byte is transmitted.
The TMIFR1 bit will be cleared automatically – once the BDLC has successfully transmitted the CRC
byte and EOD symbol – by the detection of an error on the multiplex bus or by a transmitter underrun
caused when the programmer does not write another byte to the BDR after the TDRE interrupt.
If the TMIFR1 bit is set, the BDLC will attempt to transmit the normalization symbol followed by the byte
in the BDR. After the byte in the BDR has been loaded into the transmit shift register, a TDRE interrupt
(see 4.6.4 BDLC State Vector Register) will occur similar to the main message transmit sequence. The
programmer should then load the next byte of the IFR into the BDR for transmission. When the last
byte of the IFR has been loaded into the BDR, the programmer should set the TEOD bit in the BDLC
control register 2 (BCR2). This will instruct the BDLC to transmit a CRC byte once the byte in the BDR
is transmitted and then transmit an EOD symbol, indicating the end of the IFR portion of the message
frame.
However, if the programmer wishes to transmit a single byte followed by a CRC byte, the programmer
should load the byte into the BDR before the EOD symbol has been received, and then set the TMIFR1
bit. Once the TDRE interrupt occurs, the programmer should then set the TEOD bit in the BCR2. This
will result in the byte in the BDR being the only byte transmitted before the IFR CRC byte, and no TDRE
interrupt will be generated.
If the programmer attempts to set the TMIFR1 bit immediately after the EOD symbol has been received
from the bus, the TMIFR1 bit will remain in the reset state, and no attempt will be made to transmit an
IFR byte.
If a loss of arbitration occurs when the BDLC is transmitting any byte of a multiple byte IFR, the BDLC
will go to the loss of arbitration state, set the appropriate flag, and cease transmission.
If the BDLC loses arbitration during the IFR, the TMIFR1 bit will be cleared and no attempt will be made
to retransmit the byte in the BDR. If loss of arbitration occurs in the last two bits of the IFR byte, two
additional 1 bits will be sent out.
NOTE
The extra logic 1s are an enhancement to the J1850 protocol which forces
a byte boundary condition fault. This is helpful in preventing noise from
going onto the J1850 bus from a corrupted message.
TMIFR0 — Transmit Multiple Byte IFR without CRC (Type 3) Bit
The TMIFR0 bit is used to request the BDLC to transmit the byte in the BDLC data register (BDR) as
the first byte of a multiple byte IFR without CRC. Response IFR bytes are still subject to J1850
message length maximums. See 4.4.2 J1850 Frame Format and Figure 4-19.
1 = If this bit is set prior to a valid EOD being received with no CRC error, once the EOD symbol
has been received the BDLC will attempt to transmit the appropriate normalization bit followed
by IFR bytes. The programmer should set TEOD after the last IFR byte has been written into
the BDR register. After TEOD has been set, the last IFR byte to be transmitted will be the last
byte which was written into the BDR register.
MC68HC908AS32A Data Sheet, Rev. 2.0
Freescale Semiconductor
91