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dsPIC30F6011/6012/6013/6014

DS70117F-page 114

2006 Microchip Technology Inc.

17.4

Message Reception

17.4.1

RECEIVE BUFFERS

The CAN bus module has 3 receive buffers. However,

one of the receive buffers is always committed to mon-

itoring the bus for incoming messages. This buffer is

called the Message Assembly Buffer (MAB). So there

are 2 receive buffers visible, RXB0 and RXB1, that can

essentially

instantaneously

receive

complete

message from the protocol engine.

All messages are assembled by the MAB and are trans-

ferred to the RXBn buffers only if the acceptance filter

criterion are met. When a message is received, the

RXnIF flag (CiINTF<0> or CiINRF<1>) will be set. This

bit can only be set by the module when a message is

received. The bit is cleared by the CPU when it has com-

pleted processing the message in the buffer. If the

RXnIE bit (CiINTE<0> or CiINTE<1>) is set, an interrupt

will be generated when a message is received.

RXF0 and RXF1 filters with RXM0 mask are associated

with RXB0. The filters RXF2, RXF3, RXF4, and RXF5

and the mask RXM1 are associated with RXB1.

17.4.2

MESSAGE ACCEPTANCE FILTERS

The message acceptance filters and masks are used to

determine if a message in the message assembly

buffer should be loaded into either of the receive buff-

ers. Once a valid message has been received into the

Message Assembly Buffer (MAB), the identifier fields of

the message are compared to the filter values. If there

is a match, that message will be loaded into the

appropriate receive buffer.

The acceptance filter looks at incoming messages for

the RXIDE bit (CiRXnSID<0>) to determine how to

compare the identifiers. If the RXIDE bit is clear, the

message is a standard frame and only filters with the

EXIDE bit (CiRXFnSID<0>) clear are compared. If the

RXIDE bit is set, the message is an extended frame,

and only filters with the EXIDE bit set are compared.

Configuring the RXM<1:0> bits to ‘01’ or ‘10’ can

override the EXIDE bit.

17.4.3

MESSAGE ACCEPTANCE FILTER

MASKS

The mask bits essentially determine which bits to apply

the filter to. If any mask bit is set to a zero, then that bit

will automatically be accepted regardless of the filter

bit. There are 2 programmable acceptance filter masks

associated with the receive buffers, one for each buffer.

17.4.4

RECEIVE OVERRUN

An overrun condition occurs when the Message

Assembly Buffer (MAB) has assembled a valid

received message, the message is accepted through

the acceptance filters, and when the receive buffer

associated with the filter has not been designated as

clear of the previous message.

The overrun error flag, RXnOVR (CiINTF<15> or

CiINTF<14>), and the ERRIF bit (CiINTF<5>) will be

set and the message in the MAB will be discarded.

If the DBEN bit is clear, RXB1 and RXB0 operate inde-

pendently. When this is the case, a message intended

for RXB0 will not be diverted into RXB1 if RXB0 con-

tains an unread message and the RX0OVR bit will be

set.

If the DBEN bit is set, the overrun for RXB0 is handled

differently. If a valid message is received for RXB0 and

RXFUL = 1 indicates that RXB0 is full and RXFUL = 0

indicates that RXB1 is empty, the message for RXB0

will be loaded into RXB1. An overrun error will not be

generated for RXB0. If a valid message is received for

RXB0 and RXFUL = 1, indicating that both RXB0 and

RXB1 are full, the message will be lost and an overrun

will be indicated for RXB1.

17.4.5

RECEIVE ERRORS

The CAN module will detect the following receive

errors:

Cyclic Redundancy Check (CRC) Error

Bit Stuffing Error

Invalid Message Receive Error

These receive errors do not generate an interrupt.

However, the receive error counter is incremented by

one in case one of these errors occur. The RXWAR bit

(CiINTF<9>) indicates that the receive error counter

has reached the CPU warning limit of 96 and an

interrupt is generated.

17.4.6

RECEIVE INTERRUPTS

Receive interrupts can be divided into 3 major groups,

each including various conditions that generate

interrupts:

Receive Interrupt:

A message has been successfully received and

loaded into one of the receive buffers. This inter-

rupt is activated immediately after receiving the

End of Frame (EOF) field. Reading the RXnIF flag

will indicate which receive buffer caused the

interrupt.

Wake-up Interrupt:

The CAN module has woken up from Disable

mode or the device has woken up from Sleep

mode.

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