Datasheet

 2003-2019 Microchip Technology Inc. DS20001801J-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 1,1
operation.
12.2 RESET Instruction
The RESET instruction can be used to reinitialize the inter-
nal registers of the MCP2515 and set the 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 the
CS pin low,
sending the instruction byte and then raising the
CS
pin. 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 is automatically incre-
mented 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 provide
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) provides
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, RXnIF (CANINTF), 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 continu-
ing 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. As 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 TXREQ bit (TXBnCTRL[3])
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.
Note: The MCP2515 expects the first byte after
lowering
CS to be the instruction/command
byte. This implies that
CS must be raised
and then lowered again to invoke another
command.