RXB0D2 datasheet(59/92 Pages) MICROCHIP

59 / 92 page

 2003-2012 Microchip Technology Inc.

DS21801G-page 59

MCP2515

10.0

MODES OF OPERATION

The MCP2515 has five modes of operation. These

modes are:

Configuration mode

Normal mode

Sleep mode

Listen-Only mode

Loopback mode

The

operational

mode

selected

via

the

CANCTRL. REQOP bits (see Register 10-1).

When changing modes, the mode will not actually

change until all pending message transmissions are

complete. The requested mode must be verified by

reading

the

CANSTAT.OPMODE

bits

(see

10.1

Configuration Mode

The MCP2515 must be initialized before activation.

This is only possible if the device is in the Configuration

mode. Configuration mode is automatically selected

after power-up, a Reset or can be entered from any

other mode by setting the CANTRL.REQOP bits to

‘100’. When Configuration mode is entered, all error

counters are cleared. Configuration mode is the only

mode where the following registers are modifiable:

• CNF1, CNF2, CNF3

• TXRTSCTRL

• Filter registers

• Mask registers

10.2

Sleep Mode

The MCP2515 has an internal Sleep mode that is used

to minimize the current consumption of the device. The

SPI interface remains active for reading even when the

MCP2515 is in Sleep mode, allowing access to all

registers.

To enter Sleep mode, the mode request bits are set in

the

CANCTRL

(REQOP<2:0>).

The

CANSTAT.OPMODE bits indicate operation mode.

These bits should be read after sending the Sleep

command to the MCP2515. The MCP2515 is active

and has not yet entered Sleep mode until these bits

indicate that Sleep mode has been entered.

When in internal Sleep mode, the wake-up interrupt is

still active (if enabled). This is done so that the MCU

can also be placed into a Sleep mode and use the

MCP2515 to wake it up upon detecting activity on the

bus.

When in Sleep mode, the MCP2515 stops its internal

oscillator. The MCP2515 will wake-up when bus activity

occurs or when the MCU sets, via the SPI interface, the

CANINTF.WAKIF bit to ‘generate’ a wake-up attempt

(the CANINTE.WAKIE bit must also be set in order for

the wake-up interrupt to occur).

The TXCAN pin will remain in the recessive state while

the MCP2515 is in Sleep mode.

10.2.1

WAKE-UP FUNCTIONS

The device will monitor the RXCAN pin for activity while

it is in Sleep mode. If the CANINTE.WAKIE bit is set,

the device will wake-up and generate an interrupt.

Since the internal oscillator is shut down while in Sleep

mode, it will take some amount of time for the oscillator

to start-up and the device to enable itself to receive

messages. This Oscillator Start-up Timer (OST) is

defined as 128 TOSC.

The device will ignore the message that caused the

wake-up from Sleep mode, as well as any messages

that occur while the device is ‘waking up’. The device

will wake-up in Listen-Only mode. The MCU must set

Normal mode before the MCP2515 will be able to

communicate on the bus.

The device can be programmed to apply a low-pass

filter function to the RXCAN input line while in internal

Sleep mode. This feature can be used to prevent the

device from waking up due to short glitches on the CAN

bus lines. The CNF3.WAKFIL bit enables or disables

the filter.

10.3

Listen-Only Mode

Listen-Only mode provides a means for the MCP2515

to receive all messages (including messages with

errors) by configuring the RXBnCTRL.RXM<1:0> bits.

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 at least

two other nodes are communicating with each other.

The baud rate can be detected empirically by testing

different values until valid messages are received.

Listen-Only mode is a silent mode, meaning no

messages will be transmitted while in this mode

(including error flags or acknowledge signals). In

Listen-Only mode, both valid and invalid messages will

be received regardless of filters and masks or RXMn

Receive Buffer mode bits. The error counters are reset

and deactivated in this state. The Listen-Only mode is

activated by setting the mode request bits in the

CANCTRL register.

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