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An overrun condition can occur on transmission if the transmit buffer,
JTX_BUF, is overwritten.
8.5.2
Receiving Messages
For a message reception, after a SOF is detected on the bus, the controller starts to shift data sym-
bols into the J1850 receive buffer (JRX_BUF) until an entire data byte has been received. This
byte is automatically transferred into the J1850 receive (J_RX) register (Figure 8-14) and the sub-
sequent byte is written into the empty JRX_BUF.
J_RX
The J1850 receiver (J_RX) register transfers received data in byte increments from the J1850 bus to
the microcontroller CPU. This register is buffered to allow for reception of a second data byte while
the first data byte is being read. This byte register can be read or written, and is addressable through
windowing .
7
Bit
Bit
Number
Mnemonic
7:0
DB7:0
The transfer of the first byte to J_RX triggers the reception process and generates the J1850 re-
ception (J1850RX) interrupt (if it is enabled), signaling that JRX_BUF is available for another
byte (Figure 8-15).
CPU
After J_RX is read, the byte residing in JRX_BUF is automatically shifted into J_RX, freeing
JRX_BUF for another reception. This process continues until an end of data (EOD) symbol is en-
countered.
NOTE
Receive Byte
Data Bits
These eight bits compose the last data byte received from the J1850 bus.
Figure 8-14. J1850 Receiver (J_RX) Register
JRX_BUF
J_RX
Figure 8-15. J1850 Receive Message Structure
J1850 COMMUNICATIONS CONTROLLER
Address:
Reset State:
Function
J1850 Bus
Message receive
interrupt (J1850RX) set
1F52H
00H
0
A5236-01
8-15
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