ST92F124 datasheet(308/429 Pages) STMICROELECTRONICS

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J1850 Byte Level Protocol Decoder (JBLPD)

J1850 BYTE LEVEL PROTOCOL DECODER (Cont’d)

Bit 7:4 = MLC[3:0] Message Length Count.

Message Length Count bits 3 to 0 are written when

the program writes one of the IFR opcodes. Upon

detection of the EOD symbol which delineates the

body of a frame from the IFR portion of the frame,

the received byte counter is compared against the

count contained in MLC[3:0]. If they match, then

the IFR will be transmitted. If they do not match,

then the TRA bit in the ERROR register is set and

no transmit attempt occurs.

– While NFL=0, an MCL[3:0] decimal value be-

tween 1 and 11 is considered valid. MCL[3:0]

values of 12, 13, 14, 15 are considered invalid

and will set the Transmit Request Aborted

(TRA) bit in the ERROR register.

– While NFL=1, an MCL[3:0] value between 1 and

15 is considered valid.

– For NFL=1 or 0, MCL[3:0] bits are don’t care dur-

ing a MSG or MSG+CRC opcode write.

– If writing an IFR opcode and MCL[3:0]=0000,

then the message length count check is ignored

(i.e. MLC=Count is disabled), and the IFR is en-

abled only on a correct CRC and a valid EOD

symbol assuming no other error conditions

(IFD, IBD, RBRK) appear.

Bit 3 = Reserved.

Bit 2:0 = OP[2:0] Transmit Opcode Select Bits.

The bits OP[2:0] form the code that the transmitter

uses to perform a transmit sequence. The codes

are listed in Table 58.

Table 58. Opcode definitions

MSG, Message Byte Opcode.

The Message byte opcode is set when the user

program wants to initiate or continue transmitting

the body of a message out the VPWO pin.

The body of a message is the string of data bytes

following an SOF symbol, but before the first EOD

symbol in a frame. If the J1850 bus is in an idle

condition when the opcode is written, an SOF

symbol is transmitted out the VPWO pin immedi-

ately before it transmits the data contained in TX-

DATA. If the JBLPD is not in idle and the J1850

transmitter has not been locked out by loss of arbi-

tration, then the TXDATA byte is transferred to the

serial output shift register for transmission immedi-

ately on completion of any previously transmitted

data. The final byte of a message string is not

transmitted using the MSG opcode (use the

MSG+CRC opcode).

Special Conditions for MSG Transmit:

– 1) A MSG cannot be queued on top of an execut-

ing IFR3 opcode. If so, then TRA is set, and

TDUF will get set because the transmit state

machine will be expecting more data, then the

inverted CRC is appended to this frame. Also,

no message byte will be sent on the next frame.

– 2) If NFL = 0 and an MSG queued without CRC

on Received Byte Count for this frame=10 will

trigger the TRA to get set, and TDUF will get set

because the state machine will be expecting

more data and the transmit machine will send

the inverted CRC after the byte which is pres-

ently transmitting. Also, no message byte will be

sent on the next frame.

Caution should be taken when TRA gets set in

these cases because the TDUF error sequence

may engage before the user program has a

chance to rewrite the TXOP register with the cor-

rect opcode. If a TDUF error occurs, a subsequent

MSG write to the TXOP register will be used as the

first byte of the next frame.

OP[2:0]

Transmit opcode

Abbreviation

000

No operation or

Cancel

CANCEL

001

Send Break Symbol

SBRK

010

Message Byte

MSG

011

Message Byte then ap-

pend CRC

MSG+CRC

100

In-Frame Response Type

IFR1

101

In-Frame Response Type

IFR2

110

In-Frame Response Type

IFR3

111

IFR Type 3 then append

CRC

IFR3+CRC

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