MCP25020ISLASHP datasheet(11/66 Pages) MICROCHIP

11 / 66 page

DS21664D-page 11

MCP2502X/5X

2.5

Buffers, Masks, and Filters

This part of the CAN module supports the transmitting,

receiving and acceptance of CAN messages.

Three transmit buffers are used for the three transmit

message IDs, as discussed later in this section. Two

receive buffers store the CAN message’s arbitration

field, control field and the data field.

One mask defines which bits are to be applied to either

filter. The mask can be regarded as defining “don’t

care” bits for the filter.

Each of the two filters define a bit pattern that will be

compared to all incoming messages. All filter bits that

have not been defined as “don’t care” by the mask are

applied to the message.

2.5.1

TRANSMIT MESSAGE ID’S

The MCP2502X/5X device contains three separate

transmit message ID’s: TXID0, TXID1 and TXID2. The

data length code is predefined for each of the various

output messages, with the data that is transmitted

coming directly from the contents of the device’s

peripheral registers.

2.5.1.1

Transmit Message ID0 (TXID0)

TXID0 contains the identifier that is used when trans-

mitting

the

Bus

message.

enabled

(STCON.STEN = 1), the On Bus message will be

transmitted at predefined intervals. Depending on the

message-select bit (STCON.STMS = 1), the CAN

message will send GPIO and A/D data.

Transmit Message ID0 will not automatically be sent

when the device is brought out of sleep.

2.5.1.2

Transmit Message ID1 (TXID1)

TXID1 contains the identifier that is used when the

MCP2502X/5X sends the Command Acknowledge

message, the Receive Overflow message and/or the

Error Condition message. All message types use the

same identifier.

The CAEN bit, in the OPTREG2 register, selects

between the Command Acknowledge and Receive

Overflow operation. These message types have a DLC

of 0 and do not contain any data. The Error Condition

message can occur anytime, has a DLC of 3 and

contains the EFLG, TEC and REC data values.

Command Acknowledge: TXID1 sends a Command

Acknowledge message when the MCP2502X/5X

receives an Input Message and processes the

instruction (and OPTREG2.CAEN = 1). This message

is used as a hand shake for the node requesting the

modification of the MCP2502X/5X. There is no data

associated with this message.

Receive Overflow: TXID1 sends a Receive Overflow

message if there is a Receive Overflow condition (and

OPTREG2.CAEN = 0). This only occurs if the device

has received a valid message before processing the

previous valid message from the same receive buffer.

There is no data associated with this message.

Error Condition: An Error Condition message is

transmitted if the TEC or REC counters reach error

warning (> 95) or error passive (> 127). This message

contains the TXID1 identifier and the TEC, REC and

EFLG counters.

A hysteresis is implemented in hardware that prevents

messages from repeatedly being transmitted due to

error counts changing by one or two bits. Once a

message is sent for an error warning (TEC or REC >

95), the message will not trigger again until the error

counter

≤ 79 and back to > 95 (hysteresis = 17 counts).

Similarly, an error passive message is sent at TEC or

REC > 127 and is not sent again until the error counter

≤ 111 and back to >127 (hysteresis = 17 counts).

2.5.1.3

Transmit Message ID2 (TXID2)

Transmit ID2 contains the identifier that is used when

transmitting

auto-conversion-initiated

messages,

including digital input edge detection and/or analog

input exceeding a threshold. This message will also be

sent when the device wakes up from sleep due to a

digital input change-of-state condition (i.e., change-of-

state occurs on input configured to transmit on

change-of-state).

2.6

Receive Buffers

The MCP2505X contains two receive buffers, each

with their own filter. There is also a Message Assembly

Buffer (MAB) that acts as a third receive buffer (see

Figure 2-1).

The two receive buffers, combined with the MAB help,

insure that received messages will be processed while

minimizing the chances of receive buffer overrun due to

maximum bus loading of messages destined for the

MCP2502X/5X.

Note:

A zero data length On Bus message will be

transmitted

once

after

power-up,

regardless of scheduled transmission-

enable status.

Note:

The receive buffers are used by the

MCP2502X/5X to implement the command

messages

and

are

not

externally

accessible.

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