J1850 Byte Level Protocol Decoder (JBLPD)
J1850 BYTE LEVEL PROTOCOL DECODER (Cont’d)
IFR3+CRC, In-Frame Response Type 3 then ap-
pend CRC opcode.
The IFR3 byte is attempted according to the pro-
cedure described in section “Transmitting a type 3
IFR”. The CRC byte is transmitted out on comple-
tion of the transmit of the IFR3 byte.
If this opcode sets up the last byte in an IFR3 data
string, then the TXDATA register contents shall be
transmitted out immediately upon completion of
the previous IFR3 data byte followed by the trans-
mit of the CRC byte. In this case the IFR3+CRC is
enabled on conditions 4 and 5 listed above. Note
that if an IFR3+CRC opcode is written, a queued
MSG or MSG+CRC is overridden by the
IFR3+CRC.
The In-frame Response Type 3 then append CRC
opcode (IFR3+CRC) is set if the user program
wants to either initiate to transmit a single data
byte IFR3 followed by a CRC, or transmit the last
data byte of an IFR3 string followed by the CRC
byte in response to a message that is currently be-
ing received.
The IFR3+CRC opcode transmits the contents of
the TXDATA register followed by the computed
CRC byte. The user program decides to set up an
IFR3 upon receiving a certain portion of the data
byte string of an incoming message. A previous
write of the TXDATA register should have oc-
curred.
SBRK, Send Break Symbol.
The SBRK opcode is written to transmit a nominal
break (BRK) symbol out the VPWO pin. A Break
symbol can be initiated at any time. Once the
SBRK opcode is written a BRK symbol of the nom-
inal Tv5 duration will be transmitted out the VPWO
pin immediately. To terminate the transmission of
an in-progress break symbol the JE bit should be
set to a logic zero. An SBRK command is non-
maskable, it will override any present transmit op-
eration, and it does not wait for the present trans-
mit to complete. Note that in the 4X mode a SBRK
will send a break character for the nominal Tv5
time times four (4 x Tv5) so that all nodes on the
bus will recognize the break. A CANCEL opcode
does not override a SBRK command.
The J1850 block will enable the transmission of
the first byte of an IFR3 string on these conditions:
– 1) The CRC check is valid (otherwise the CRCE
is set)
– 2) The received message length is valid if ena-
bled (otherwise the TRA is set)
– 3) A valid EOD minimum symbol is received (oth-
erwise the IFD may eventually get set due to
byte synchronization errors)
– 4) If NFL = 0 & Received Byte Count for this
frame <=10 (otherwise TRA is set and inverted
CRC is transmitted)
CANCEL, No Operation or Cancel Pending Trans-
mit.
– 5) If not presently executing an MSG opcode
(otherwise TRA is set, and TDUF will get set be-
cause the transmit state machine will be expect-
ing more data and the inverted CRC will be
appended to this frame)
The Cancel opcode is used by the user program to
tell the J1850 transmitter that a previously queued
opcode should not be transmitted. The Cancel op-
code will set the TRDY bit. If the JBLPD peripheral
is presently not transmitting, the Cancel command
effectively cancels a pending MSGx or IFRx op-
code if one was queued, or it does nothing if no
opcode was queued. If the JBLPD peripheral is
presently transmitting, then a queued MSGx or
IFRx opcode is aborted and the TDUF circuit may
take affect.
– 6) If not presently executing an IFR1, IFR2 or
IFR3+CRC opcodes, otherwise TRA is set (but
no TDUF)
– 7) If not presently receiving an IFR portion of a
frame, otherwise TRA is set.
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STMICROELECTRONICS [ ST ]
