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J1850 Byte Level Protocol Decoder (JBLPD)

J1850 BYTE LEVEL PROTOCOL DECODER (Cont’d)

10.9.3.2 Transmitting Messages

This section describes the general procedures

used by the JBLPD to successfully transmit J1850

frames of data out the VPWO pin. The first five

sub-sections describe the procedures used for

transmitting the specific transmit data types. The

last section goes into the details of the transmitted

symbol timing, synchronizing of symbols received

from the external J1850 bus, and how data bit ar-

bitration works.

The important concept to note for transmitting data

is: the activity sent over the VPWO line should be

timed with respect to the levels and transitions

seen on the filtered VPWI line.

The J1850 bus is a multiplexed bus, and the

VPWO & VPWI pins interface to this bus through a

transceiver I.C. Therefore, the propagation delay

through the transceiver I.C. and external bus filter-

ing must be taken into account when looking for

transmitted edges to appear back at the receiver.

The external propagation delay for an edge sent

out on the VPWO line, to be detected on the VPWI

line is denoted as Tp-ext and is programmable be-

tween 0 and 31 µs nominal via the JDLY[4:0] bits

in CONTROL register.

The transmitter VPW encoder sets the proper level

to be sent out the VPWO line. It then waits for the

corresponding level transition to be reflected back

at the VPW decoder input.

Taking into account the external loop delay (Tp-ext)

and the digital filter delay, the encoder will time its

output to remain at this level so that the received

symbol is at the correct nominal symbol time (refer

to “Transmit Opcode Queuing” section). If arbitra-

tion is lost at any time during bit 0 or bit 1 transmis-

sion, then the VPWO line goes passive. At the end

of the symbol time on VPWO, the encoder chang-

es the state of VPWO if any more information is to

be transmitted. It then times the new state change

from the receiver decoder output.

Note that depending on the symbol (especially the

SOF, NB0, NB1 symbols) the decoder output may

actually change to the desired state before the

transmit is attempted. It is important to still syn-

chronize off the decoder output to time the VPWO

symbol time.

A detailed description of the JBLPD opcodes can

be find in the description of the OP[2:0] bits in the

TXOP register.

Message Byte String Transmission (Type 0

IFR)

Message byte transmitting is the outputting of data

bytes on the VPWO pin that occurs subsequent to

a received bus idle condition. All message byte

strings start with a SOF symbol transmission, then

one or more data bytes are transmitted. A CRC

byte is then transmitted followed by an EOD sym-

bol (see Figure 131) to complete the transmission.

If transmission is queued while another frame is

being received, then the JBLPD will time an Inter-

Frame Separation (IFS) time (Tv6) before com-

mencing with the SOF character.

The user program will decide at some point that it

wants to initiate a message byte string. The user

program writes the TXDATA register with the first

message data byte to be transmitted. Next, the

TXOP register is written with the MSG opcode if

more than one data byte is contained within the

message, or with MSG+CRC opcode if one data

byte is to be transmitted. The action of writing the

TXOP register causes the TRDY bit to be cleared

signifying that the TXDATA register is full and a

corresponding opcode has been queued. The

JBLPD must wait for an EOF nominal time period

at which time data is transferred from the TXDATA

is again set since the TXDATA register is empty.

The JBLPD should also begin transmission if an-

other device begins transmitting early. As long as

an EOF minimum time period elapses, the JBLPD

should begin timing and asserting the SOF symbol

with the intention of arbitrating for the bus during

the transmission of the first data byte. If a transmit

is requested during an incoming SOF symbol, the

JBLPD should be able to synchronize itself to the

incoming SOF up to a time of Tv1 max. (96 µs) into

the SOF symbol before declaring that it was too

late to arbitrate for this frame.

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