J1850 Byte Level Protocol Decoder (JBLPD)
J1850 BYTE LEVEL PROTOCOL DECODER (Cont’d)
If the J1850 bus was IDLE at the time the first data
byte and opcode are written, the transmitter will
immediately transfer data from the TXDATA regis-
ter to the transmit shift register. The TRDY bit will
once again be set signifying the readiness to ac-
cept a new data byte. The second data byte can
then be written followed by the respective opcode.
In the case of the last data byte, the TXOP register
should be written with the MSG+CRC opcode. The
transmitter will transmit the internally generated
CRC after the last bit of the data byte. Once the
TRDY bit is set signifying the acceptance of the
last data byte, the first byte of the next message
can be queued by writing the TXDATA register fol-
lowed by a TXOP register write. The block will wait
until the current data and the CRC data byte are
sent out and a new IFS has expired before trans-
mitting the new data. This is the case even if IFR
data reception takes place in the interim.
register except during DMA transfers (see Section
10.9.6.4 DMA Management in Transmission
Mode).
Transmitting a Type 1 IFR
The user program will decide to transmit an IFR
type 1 byte in response to a message which is cur-
rently being received (See Figure 132). It does so
by writing the IFR1 opcode to the TXOP register.
Transmitting IFR data type 1 requires only a single
write of the TXOP register with the IFR1 opcode
set. The MLC[3:0] bits should be set to the proper
“byte-received-count-required-before-IFR’ing” val-
ue. If no error conditions (IBD, IFD, TRA, RBRK or
CRCE) exist to prevent transmission, the JBLPD
peripheral will then transmit out the contents of the
PADDR register at the next EOD nominal time pe-
riod or at a time greater than the EOD minimum
time period if a falling edge is detected on filtered
J1850 bus line signifying another transmitter is be-
ginning early. The NB1 symbol precedes the PAD-
DR register value and is followed with an EOF de-
limiter. The TRDY flag is cleared on the write of the
TXOP register. The TRDY bit is set once the NB1
begins transmitting.
Lost arbitration any time during the transfer of type
0 data will be honoured by immediately relinquish-
ing control to the higher priority message. The TLA
bit in the STATUS register is set accordingly and
an interrupt will be generated assuming the
TLA_M bit in the IMR register is set. It is responsi-
bility of the user program to re-send the message
beginning with the first byte if desired. This may be
done at any time by rewriting only the TXOP regis-
ter if the TXDATA contents have not changed.
Although the JBLPD should never lose arbitration
for data in the IFR portion of a type 1 frame, higher
priority messages are always honoured under the
rules of arbitration. If arbitration is lost then the
VPWO line is set to the passive state. The TLA bit
in the STATUS register is set accordingly and an
interrupt will be generated if enabled. The IFR1 is
not retried. It is lost if the JBLPD peripheral loses
arbitration. Also, the data that made it out on the
bus will be received in the RXDATA register if not
put into sleep mode. Note that for the transmitter to
synchronize to the incoming signals of a frame, an
IFR should be queued before an EODM is re-
ceived for the present frame.
Any transmitted data and CRC bytes during the
transmit frame will also be received and trans-
ferred to the RXDATA register if the corresponding
message filter bit is set in the FREG[0:31] regis-
ters. If the corresponding bit is not set in
FREG[0:31], then the transmitted data is also not
transferred to RXDATA. Also, the RDRF will not
get set during frame and receive events such as
RDOF & EODM.
NOTE: The correct procedure for transmitting is to
write first the TXDATA register and then the TXOP
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STMICROELECTRONICS [ ST ]
