ST92F124 datasheet(311/429 Pages) STMICROELECTRONICS

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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 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.

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)

– 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)

– 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.

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

MSG+CRC

overridden

the

IFR3+CRC.

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.

CANCEL, No Operation or Cancel Pending Trans-

mit.

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.

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