RXM0SIDL datasheet(65/92 Pages) MICROCHIP

65 / 92 page

 2003-2012 Microchip Technology Inc.

DS21801G-page 65

MCP2515

12.0

SPI INTERFACE

12.1

Overview

The MCP2515 is designed to interface directly with the

Serial Peripheral Interface (SPI) port available on many

microcontrollers and supports Mode 0,0 and Mode 1,1.

Commands and data are sent to the device via the SI

pin, with data being clocked in on the rising edge of

SCK. Data is driven out by the MCP2515 (on the SO

line) on the falling edge of SCK. The CS pin must be

held low while any operation is performed. Table 12-1

shows the instruction bytes for all operations. Refer to

Figure 12-10 and Figure 12-11 for detailed input and

output timing diagrams for both Mode 0,0 and Mode

,1 operation.

12.2

RESET

Instruction

The RESET instruction can be used to re-initialize the

internal registers of the MCP2515 and set Configuration

mode. This command provides the same functionality,

via the SPI interface, as the RESET pin.

The RESET instruction is a single-byte instruction that

requires selecting the device by pulling CS low,

sending the instruction byte and then raising CS. It is

highly recommended that the Reset command be sent

(or the RESET pin be lowered) as part of the power-on

initialization sequence.

12.3

READ

Instruction

The READ instruction is started by lowering the CS pin.

The READ instruction is then sent to the MCP2515

followed by the 8-bit address (A7 through A0). Next, the

data stored in the register at the selected address will

be shifted out on the SO pin.

The

internal

Address

Pointer

automatically

incremented to the next address once each byte of

data is shifted out. Therefore, it is possible to read the

next consecutive register address by continuing to pro-

vide clock pulses. Any number of consecutive register

locations can be read sequentially using this method.

The read operation is terminated by raising the CS pin

(Figure 12-2).

12.4

READ RX BUFFER

Instruction

The READ RX BUFFER instruction (Figure 12-3) pro-

vides a means to quickly address a receive buffer for

reading. This instruction reduces the SPI overhead by

one byte, the address byte. The command byte actually

has four possible values that determine the Address

Pointer location. Once the command byte is sent, the

controller clocks out the data at the address location

the same as the READ instruction (i.e., sequential reads

are possible). This instruction further reduces the SPI

overhead by automatically clearing the associated

receive flag (CANINTF.RXnIF) when CS is raised at the

end of the command.

12.5

WRITE

Instruction

The WRITE instruction is started by lowering the CS

pin. The WRITE instruction is then sent to the MCP2515

followed by the address and at least one byte of data.

It is possible to write to sequential registers by

continuing to clock in data bytes, as long as CS is held

low. Data will actually be written to the register on the

rising edge of the SCK line for the D0 bit. If the CS line

is brought high before eight bits are loaded, the write

will be aborted for that data byte and previous bytes in

the command will have been written. Refer to the timing

diagram in Figure 12-4 for a more detailed illustration of

the byte write sequence.

12.6

LOAD TX BUFFER

Instruction

The LOAD TX BUFFER instruction (Figure 12-5) elimi-

nates the eight-bit address required by a normal Write

command. The eight-bit instruction sets the Address

Pointer to one of six addresses to quickly write to a

transmit buffer that points to the “ID” or “data” address

of any of the three transmit buffers.

12.7

REQUEST-TO-SEND

(RTS)

Instruction

The RTS command can be used to initiate message

transmission for one or more of the transmit buffers.

The MCP2515 is selected by lowering the CS pin. The

RTS command byte is then sent. Shown in Figure 12-6,

the last 3 bits of this command indicate which transmit

buffer(s) are enabled to send.

This command will set the TxBnCTRL.TXREQ bit for

the respective buffer(s). Any or all of the last three bits

can be set in a single command. If the RTS command

is sent with nnn = 000, the command will be ignored.

12.8

READ STATUS

Instruction

The READ STATUS instruction allows single instruction

access to some of the often used status bits for

message reception and transmission.

The MCP2515 is selected by lowering the CS pin and

the Read Status command byte, shown in Figure 12-8,

is sent to the MCP2515. Once the command byte is

sent, the MCP2515 will return eight bits of data that

contain the status.

If additional clocks are sent after the first eight bits are

transmitted, the MCP2515 will continue to output the

status bits as long as the CS pin is held low and clocks

are provided on SCK.

Note:

The MCP2515 expects the first byte after

lowering

the

instruction/

command byte. This implies that CS must

be raised and then lowered again to

invoke another command.

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