PIC18F258-ESLASHLSQTP datasheet(228/402 Pages) MICROCHIP

228 / 402 page

PIC18FXX8

DS41159E-page 226

19.3

CAN Modes of Operation

The PIC18FXX8 has six main modes of operation:

• Configuration mode

• Disable mode

• Normal Operation mode

• Listen Only mode

• Loopback mode

• Error Recognition mode

All modes, except Error Recognition, are requested by

setting the REQOP bits (CANCON<7:5>); Error Recog-

nition is requested through the RXM bits of the Receive

Buffer register(s). Entry into a mode is Acknowledged

by monitoring the OPMODE bits.

When changing modes, the mode will not actually

change until all pending message transmissions are

complete. Because of this, the user must verify that the

device has actually changed into the requested mode

before fUrther Operations Are Executed.

19.3.1

CONFIGURATION MODE

The CAN module has to be initialized before the

activation. This is only possible if the module is in the

Configuration mode. The Configuration mode is

requested by setting the REQOP2 bit. Only when the

OPMODE2 status bit has a high level can the initializa-

tion be performed. Afterwards, the Configuration

registers, the Acceptance Mask registers and the

Acceptance Filter registers can be written. The module

is activated by setting the REQOP control bits to zero.

The module will protect the user from accidentally

violating the CAN protocol through programming

errors. All registers which control the configuration of

the module can not be modified while the module is on-

line. The CAN module will not be allowed to enter the

Configuration mode while a transmission is taking

place. The CONFIG bit serves as a lock to protect the

following registers.

• Configuration registers

• Bus Timing registers

• Identifier Acceptance Filter registers

• Identifier Acceptance Mask registers

In the Configuration mode, the module will not transmit

or receive. The error counters are cleared and the

interrupt flags remain unchanged. The programmer will

have access to Configuration registers that are access

restricted in other modes.

19.3.2

DISABLE MODE

In Disable mode, the module will not transmit or

receive. The module has the ability to set the WAKIF bit

due to bus activity, however, any pending interrupts will

remain and the error counters will retain their value.

If REQOP<2:0> is set to ‘001’, the module will enter the

Module Disable mode. This mode is similar to disabling

other peripheral modules by turning off the module

enables. This causes the module internal clock to stop

unless the module is active (i.e., receiving or transmit-

ting a message). If the module is active, the module will

wait for 11 recessive bits on the CAN bus, detect that

condition as an IDLE bus, then accept the module

disable command. OPMODE<2:0> = 001 indicates

whether the module successfully went into Module

Disable mode.

The WAKIF interrupt is the only module interrupt that is

still active in the Module Disable mode. If the WAKIE is

set, the processor will receive an interrupt whenever

the CAN bus detects a dominant state, as occurs with

a SOF. If the processor receives an interrupt while it is

sleeping, more than one message may get lost. User

firmware must anticipate this condition and request

retransmission. If the processor is running while it

receives an interrupt, only the first message may get

lost.

The I/O pins will revert to normal I/O function when the

module is in the Module Disable mode.

19.3.3

NORMAL MODE

This is the standard operating mode of the PIC18FXX8.

In this mode, the device actively monitors all bus

messages and generates Acknowledge bits, error

frames, etc. This is also the only mode in which the

PIC18FXX8 will transmit messages over the CAN bus.

19.3.4

LISTEN ONLY MODE

Listen

Only

mode

provides

means

for

the

PIC18FXX8

receive

all

messages,

including

messages with errors. This mode can be used for bus

monitor applications or for detecting the baud rate in

‘hot plugging’ situations. For auto-baud detection, it is

necessary that there are at least two other nodes which

are communicating with each other. The baud rate can

be detected empirically by testing different values until

valid messages are received. The Listen Only mode is

a silent mode, meaning no messages will be trans-

mitted while in this state, including error flags or

Acknowledge signals. The filters and masks can be

used to allow only particular messages to be loaded

into the receive registers, or the filter masks can be set

to all zeros to allow a message with any identifier to

pass. The error counters are reset and deactivated in

this state. The Listen Only mode is activated by setting

the mode request bits in the CANCON register.

PIC18F258-E/LSQTP Datasheet(PDF) 228 Page - Microchip Technology

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