MCP2502X datasheet(21/66 Pages) MICROCHIP

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DS21664D-page 21

MCP2502X/5X

4.0

DEVICE OPERATION

4.1

Power-Up Sequence

The following sections describe the events/actions of

the MCP2502X/5X during normal power-up and

operation.

4.1.1

POWER-ON RESET

The MCP2502X/5X goes through a sequence of events

at power-on reset (POR) in order to load the

programmed configuration and insure that errors are

not introduced on the bus. During this time, the device

is prevented from generating a low condition on the

TXCAN pin. The TXCAN pin must remain high from

power-on until the device goes on bus.

Operational Mode at Power-On

The MCP2502X/5X initially powers up in Configuration

mode. While in this mode, the MCP2502X/5X will be

prevented from sending or receiving messages via the

CAN interface. The ADC and PWM peripherals are

disabled while in this mode.

Self-Configuration

Once the MCP2502X/5X is out of reset, it will perform

self-configuration.

This

accomplished

transferring the contents of the EPROM array to the

corresponding locations within the SRAM array. In

addition, the checksum of the data written to SRAM will

be compared to a pre-programmed value as a test of

valid data.

Going On Bus

Once the self-configuration cycle has successfully

completed, the MCP2502X/5X switches to Listen-only

mode. It will remain in this mode until an error-free CAN

message is detected. This is done to ensure that the

device is at the correct bus rate for the system.

Once the device detects an error-free message, it waits

for CAN bus idle before switching to Normal mode. This

prevents it from going on bus in the middle of another

node’s transmission and generating an error frame.

Alternately, the MCP2505X may directly enter Normal

mode (without first entering Listen-only Mode) after

completing its self-configuration. This is configured by

the user via a control bit (OPTREG2.PUNRM).

Once the MCP2502X/5X enters Normal mode, it is

ready to send/receive messages via the CAN interface.

At this point the ADC and PWM peripherals are

operational, if enabled.

Scheduled Transmissions

Once the MCP2502X/5X has gone on bus it will

transmit the On Bus message once, regardless of

whether enabled or not. This message notifies the

network of the MCP2502X/5X’s presence. The On Bus

message will (if enabled STCON.STEN) repeat at a

frequency

determined

the

STCON

(Register 4-1).

This message can also be configured to send the

“Read A/D Register” data bytes along with the

predefined

identifier

TXID2

setting

STCON.STMS = 1.

4.2

Message Functions

The MCP2502X/5X uses the global mask (RXMASK),

two filters (RXF0 and RXF1) and two receive buffers

(RB0 and RB1) to determine if a received message

should be acted upon. There are 16 functions that can

be performed by the MCP2502X/5X based on received

messages (see Table 4-1).These functions allow the

device to not only be accessed for Information

Request/Input/Output operations, but also to be

reconfigured via the CAN bus, if necessary.

4.3

Message Types

There are three types of messages that are used to

implement the functions of Table 4-1.

Information Request Messages (IRM)

- Received by the MCP2502X/5X.

Output Messages - Transmitted from the

MCP2502X/5X as a response to IRMs.

Input Messages - Received by the MCP2502X/

5X and used to modify registers.

Note:

The first On Bus message sent after

power-up will NOT send the “Read A/D

STCON.STMS value.

Note:

If the MCP2502X/5X enters SLEEP mode,

the scheduled transmissions will cease

until the device wakes up again. This

implies that SLEEP mode has priority over

scheduled transmissions.

Note:

Information Request Messages (IRMs)

and

Input

messages

are

both

input

messages to the MCP2502X/5X. IRMs are

received into receive buffer 0 and input

messages are received into receive buffer

1. This must be taken into account while

configuring the acceptance filters.

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