Microchip Technology dsPIC30F - Page 635

© 2004 Microchip Technology Inc. DS70070B-page 23-37
Section 23. CAN
CAN Module
23
Figure 23-7: Entering and Exiting Module Disable Mode
23.5.3 Loopback Mode
If the Loopback mode is activated, the module will connect the internal transmit signal to the
internal receive signal at the module boundary. The transmit and receive pins revert to their
PORT I/O function.
The transmitter will receive an acknowledge for its sent messages. Special hardware will
generate an acknowledge for the transmitter.
23.5.4 Listen Only Mode
Listen Only mode and Loopback modes are special cases of Normal Operation mode to allow
system debug. If the Listen Only mode is activated, the module on the CAN bus is passive. The
transmitter buffers revert to the PORT I/O function. The receive pins remain as inputs to the CAN
module. For the receiver, no error flags or Acknowledge signals are sent. The error counters are
deactivated in this state. The Listen Only mode can be used for detecting the baud rate on the
CAN bus. To use this, it is necessary that there are at least two further nodes that communicate
with each other. The baud rate can be detected empirically by testing different values. This mode
is also useful as a bus monitor without influencing the data traffic.
1 2
4 5
- Processor writes REQOP<2:0> while module receiving/transmitting message. Module continues with CAN message.
- Module detects 11 recessive bits. Module acknowledges Disable mode and sets OPMODE<2:0> bits. Module disables.
- Processor writes REQOP<2:0> during CAN bus activity. Module waits for 11 recessive bits before accepting activate.
- Module detects 11 recessive bits. Module acknowledges Normal mode and sets OPMODE<2:0> bits. Module activates.
OSC1
CAN bus
CAN Module
Disabled
3
001
001
000
000 000
000
- CAN bus message will set WAKIF bit. If WAKIE = ’ 1 ’, processor will vector to the interrupt address. CAN message ignored.
WAKIF
WAKIE
1
2
3
4
5
OPMODE<2:0>
REQOP<2:0>