MCP25020ISLASHP datasheet(23/66 Pages) MICROCHIP

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DS21664D-page 23

MCP2502X/5X

4.3.2

OUTPUT MESSAGES

The data frame sent in response to the information

request message is defined as an output message.

If the data fame is in response to a remote frame, it will

have the same identifier (standard or extended) and

contain the same number of data bytes specified by the

DLC of the remote frame (per the CAN 2.0B

specification).

If the output message is in response to a data frame,

the lower-three LSb’s of the identifier (standard or

extended) must be the same as the received message,

as well as the upper-seven MSb’s in the case of a

standard identifier, or the upper 25 MSb’s in the case of

an extended identifier. Bit 3 of a standard or extended

identifier of the output message will differ from the

received information request message in that the value

equals ‘1’ for an IRM and equals ‘0’ for the resulting

output message.

Output messages contain the requested data (in the

data

field).

Example:

The

information

request

message Read CAN error is a remote transmit request

received by the MCP2502X/5X with a DLC of 3. The

responding output message will return a data frame

that contains the same identifier (standard or extended)

as the receive message. The accompanying data bytes

will contain the values of the predefined GPIO registers

and related control/status registers, as shown in

Table 4-2 and 4-3.

4.3.3

INPUT MESSAGES

Input messages are received into receive buffer 1 and

are used to change the values of the pre-defined

groups of registers. There is also an input message

that can change a single register’s contents. The

primary purpose of input messages are to reconfigure

MCP2502X/5X parameters (if needed) while in an

operating CAN system and are, therefore, optional in

system implementation. These messages are in the

form of standard (or extended) data frames (per the

CAN 2.0B specification) that have identifiers which

pass the MCP2502X/5X’s mask/filter process (using

RXF1). After passing the mask and filter, the lower-

three

bits

the

standard

identifier

(RXF1SIDL.SID2:SID0) will indicate which register(s)

are to be written. The values for the register(s) are

contained in the data byte registers as defined in

Table 4-2.

4.4

Dynamic Message Handling

The design insures that transmit and receive messages

are

handled

properly

for

variable

bus-loading

conditions and different transmit/receive combinations.

4.4.1

MESSAGE ACCEPTANCE/

REJECTION

Messages received that meet the Mask/RXFn criteria

are then compared to the requirements for input

messages or IRMs, as determined by the filter used to

accept the message. If the message meets the

requirements of one of the associated input or

information request messages, the appropriate actions

for that message function are taken.

4.4.2

RECEIVING MULTIPLE MESSAGES

The MCP2502X/5X can only receive and process one

message at a time. While the MCP2502X/5X should

have ample time to process any received message

before another is completely received, a second

message received before the first message is finished

processing will be lost.

However, the MCP2502X/5X has the ability to notify the

network if a message is lost. TXID1 can be configured

to transmit a message if a receive overflow occurs

(OPTREG2.CAEN = 0).

Note: If the DLC of the incoming remote frame

differs

from

the

message

definitions

summarized in Table 4-2 and Table 4-3, the

resulting output message will limit itself to the

erroneous

DLC

that

was

received

(to

maintain compliance with the Bosch CAN

specification). The output message will

concatenate the number of data bytes for an

erroneous DLC that is less than the defined

number. For an erroneous DLC that is

greater

than

the

defined

number,

the

MCP2502X/5X will extend the number of

data bytes, with the data value of the last

defined data byte being repeated in the extra

bytes in the data field.

Note: If using more than one controlling node, the

MCP2502X/5X must be set up to accept

input messages with different identifiers in

order to avoid possible message collisions in

the DLC or data bytes if transmitted at the

same time.

Note 1: IRMs can theoretically be sent by more

than one controlling node because the

message is a predefined constant and

destructive collisions will not occur.

2: The number of data bytes in an input

message must match the DLC number as

defined in Table 4-2 and Table 4-3. If the

user specifies and transmits an input

message with a DLC that is less than the

required number of data bytes, the

MCP25020 will operate on corrupted data

for the bytes that it did not receive and

unknown results will occur.

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