MCP2518FD datasheet(69/96 Pages) MICROCHIP

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DS20000000A-page 69

MCP2518FD

4.2

Message Memory Access

The Message Memory (RAM) access is word-oriented

(4 bytes at a time). Any multiple of 4 data bytes can be

read or written with one instruction. The address is

incremented by one automatically after every data

byte. The address rolls over from 0xBFF to 0x400.

Writes and Reads must be word-aligned. The lower two

bits of the address are always assumed to be 0. It is not

possible to do unaligned reads/writes.

The following SPI instructions only show the different

fields and their values. Every instruction follows the

generic format illustrated in Figure 4-1.

4.2.1

MESSAGE MEMORY READ – READ

Figure 4-5 illustrates the READ instruction, while

accessing RAM. The instruction starts with nCS going

low. The Command (C<3:0> = 0b0011), is followed by

the Address (A<11:0>). Afterwards, the data byte from

address A (DB[A]) is shifted out, followed by data byte

from address A+1 (DB[A+1]). The instruction ends

when nCS goes high.

Read commands from RAM must always read a multi-

ple of 4 data bytes. A word is internally read from RAM

after the address field, and after every fourth data byte

read on the SPI. In case nCS goes high before a multi-

ple of 4 data bytes is read on SDO, the incomplete

read should be discarded by the microcontroller.

4.2.2

MESSAGE MEMORY

WRITE – WRITE

Figure 4-6 illustrates the WRITE instruction, while

accessing RAM. The instruction starts with nCS going

low. The Command (C<3:0> = 0b0010), is followed by

the Address (A<11:0>). Afterwards, the data byte is

shifted into address A (DB[A]), next into address A+1

(DB[A+1]). The instruction ends when nCS goes high.

Write commands must always write a multiple of 4

data bytes. After every fourth data byte, with the falling

edge on SCK, the RAM Word gets written. In case

nCS goes high before a multiple of 4 data bytes is

received on SDI, the data of the incomplete Word will

not be written to RAM.

FIGURE 4-5:

MESSAGE MEMORY READ INSTRUCTION

FIGURE 4-6:

MESSAGE MEMORY WRITE INSTRUCTION

DW[A]

DB[A]

DB[A+1]

DB[A+3]

DB[A+2]

0b0011

A<11:0>

nCS Low

nCS High

DW[A]

DB[A]

DB[A+1]

DB[A+3]

DB[A+2]

0b0010

A<11:0>

nCS Low

nCS High

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