RXB0D2 datasheet(23/92 Pages) MICROCHIP

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 2003-2012 Microchip Technology Inc.

DS21801G-page 23

MCP2515

4.0

MESSAGE RECEPTION

4.1

Receive Message Buffering

The MCP2515 includes two full receive buffers with

multiple acceptance filters for each. There is also a

separate Message Assembly Buffer (MAB) that acts as

a third receive buffer (see Figure 4-2).

4.1.1

MESSAGE ASSEMBLY BUFFER

Of the three receive buffers, the MAB is always

committed to receiving the next message from the bus.

The MAB assembles all messages received. These

messages will be transferred to the RXBn buffers (see

filter criteria is met.

4.1.2

RXB0 AND RXB1

The remaining two receive buffers, called RXB0 and

RXB1, can receive a complete message from the

protocol engine via the MAB. The MCU can access one

buffer, while the other buffer is available for message

reception, or for holding a previously

received

message.

4.1.3

RECEIVE FLAGS/INTERRUPTS

When a message is moved into either of the receive

buffers, the appropriate CANINTF.RXnIF bit is set. This

bit must be cleared by the MCU in order to allow a new

message to be received into the buffer. This bit

provides a positive lockout to ensure that the MCU has

finished with the message before the MCP2515

attempts to load a new message into the receive buffer.

If the CANINTE.RXnIE bit is set, an interrupt will be

generated on the INT pin to indicate that a valid

message

has

been

received.

addition,

the

associated RXnBF pin will drive low if configured as a

receive buffer full pin. See Section 4.4 “RX0BF and

RX1BF Pins” for details.

4.2

Receive Priority

RXB0, the higher priority buffer, has one mask and two

message acceptance filters associated with it. The

received message is applied to the mask and filters for

RXB0 first.

RXB1 is the lower priority buffer, with one mask and

four acceptance filters associated with it.

In addition to the message being applied to the RB0

mask and filters first, the lower number of acceptance

filters makes the match on RXB0 more restrictive and

implies a higher priority for that buffer.

When a message is received, bits <3:0> of the

RXBnCTRL register will indicate the acceptance filter

number that enabled reception and whether the

received message is a remote transfer request.

4.2.1

ROLLOVER

Additionally, the RXB0CTRL register can be configured

such that, if RXB0 contains a valid message and

another valid message is received, an overflow error

will not occur and the new message will be moved into

RXB1, regardless of the acceptance criteria of RXB1.

4.2.2

RXM BITS

The RXBnCTRL.RXM bits set special receive modes.

Normally, these bits are cleared to 00 to enable

reception of all valid messages as determined by the

appropriate acceptance filters. In this case, the

determination of whether or not to receive standard or

extended

messages

determined

the

RFXnSIDL.EXIDE bit in the acceptance filter register.

If the RXBnCTRL.RXM bits are set to 01 or 10, the

receiver will only accept messages with standard or

extended identifiers, respectively. If an acceptance

filter has the RFXnSIDL.EXIDE bit set such that it does

not correspond with the RXBnCTRL.RXM mode, that

acceptance filter is rendered useless. These two

modes of RXBnCTRL.RXM bits can be used in

systems where it is known that only standard or

extended messages will be on the bus.

If the RXBnCTRL.RXM bits are set to 11, the buffer will

receive all messages, regardless of the values of the

acceptance filters. Also, if a message has an error

before the EOF, that portion of the message assembled

in the MAB before the error frame will be loaded into the

buffer. This mode has some value in debugging a CAN

system and would not be used in an actual system

environment.

Note:

The entire content of the MAB is moved

into the receive buffer once a message is

accepted. This means, that regardless of

the

type

identifier

(standard

extended) and the number of data bytes

received, the entire receive buffer is

overwritten

with

the

MAB

contents.

Therefore, the contents of all registers in

the buffer must be assumed to have been

modified when any message is received.

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