MCP3918 datasheet(51/88 Pages) MICROCHIP

51 / 88 page

 2014 Microchip Technology Inc.

DS20005287A-page 51

MCP3918

FIGURE 6-9:

Continuous Read Sequences With and Without CRC Checksum Enabled.

The CRC checksum in the MCP3918 device uses the

16-bit CRC-16 ANSI polynomial as defined in the IEEE

802.3 standard: x16 +x15 +x2 +1. This polynomial can

also be noted as 0x8005. CRC-16 detects all single

and double-bit errors, all errors with an odd number of

bits, all burst errors of length 16 or less, and most errors

for longer bursts. This allows an excellent coverage of

the SPI communication errors that can happen in the

system,

and

heavily

reduces

the

risk

miscommunication, even under noisy environments.

The CRC-16 format displayed on the SDO pin depends

on the WIDTH_DATA<1> bit in the STATUSCOM

32-bit format to be compatible with both 16-bit and

32-bit MCUs. The CRCREG<15:0> bits calculated by

the MCP3918 device are not dependent on the format

(the device always calculates only a 16-bit CRC

checksum).

recommended

keep

WIDTH_DATA<1> = WIDTH_CRC when the CRC

checksum is enabled. If a 32-bit MCU is used in the

application, it is recommended to use 32-bit formats

(WIDTH_DATA<1> = WIDTH_CRC = 1) only.

FIGURE 6-10:

CRC Checksum Format.

The CRC computed by the MCP3918 device is fully

compatible with the CRC hardware contained in the

Direct Memory Access (DMA) peripheral of the PIC24

and PIC32 MCU product lines. The CRC message that

should be considered in the PIC® device DMA is the

concatenation of the read sequence and its associated

checksum. When the DMA CRC hardware computes

this extended message, the resulting checksum should

be 0x0000. Any other result indicates that a

miscommunication has happened and that the current

communication sequence should be stopped and

restarted.

Continuous READ communication without CRC checksum (EN_CRCCOM=0)

Don’t care

SCK

SDI

SDO

Hi-Z

CONTROL

BYTE

16x/24x/32x

Depending on

data format

ADDR

...

ADDR + n

Starts read sequence

at address ADDR

Complete READ sequence

ADDR + 1

16x/24x/32x

Depending on

data format

...

16x/24x/32x

Depending on

data format

16x/24x/32x

Depending on

data format

ADDR

...

Complete READ sequence

ADDR + 1

16x/24x/32x

Depending on

data format

...

16x/24x/32x

Depending on

data format

ADDR + n

Don’t care

ADDR

ADDR + 1

...

ADDR + n

Complete

READ

sequence

Roll-over

ADDRESS SET

Continuous READ communication with CRC checksum (EN_CRCCOM=1)

Don’t care

SCK

SDI

SDO

Hi-Z

CONTROL

BYTE

16x/24x/32x

Depending on

data format

ADDR

...

ADDR + n

Starts read sequence

at address ADDR

Complete READ sequence = Message for CRC Calculation

ADDR + 1

16x/24x/32x

Depending on

data format

...

16x/24x/32x

Depending on

data format

16x/24x/32x

Depending on

data format

ADDR

...

New Message

ADDR + 1

16x/24x/32x

Depending on

data format

...

16x/24x/32x

Depending on

data format

ADDR + n

Don’t care

ADDR

ADDR + 1

...

ADDR + n

Complete

READ

sequence

Roll-over

ADDRESS SET

CRC Checksum

16x or 32x

Depending on

CRC format

16x or 32x

Depending on

CRC format

CRC Checksum

(not part of register map)

New Checksum

Checksum

High Z

WIDTH_DATA<1> = 0

16-bit format

CRCCOM

<15:8>

CRCCOM

<7:0>

WIDTH_DATA<1> = 1

32-bit format

CRCCOM

<15:8>

CRCCOM

<7:0>

0x00

Note:

The CRC will be generated only at the end

of the selected address set, before the

rollover of the address pointer occurs (see

Figure 6-10).

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