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Data Sheet

ADFS7124-8

DIAGNOSTICS

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Rev. 0 | 71 of 93

because their contents change continuously. The CRC is performed

at a rate of 1/2400 seconds. Each time that the memory map is

accessed, the CRC is recalculated. Events that cause the CRC to

be recalculated are:

►

A user write.

►

An offset/full-scale calibration.

►

When the device is operated in the single conversion mode and

the ADC goes into the standby mode following the completion of

the conversion.

►

When exiting the continuous read mode (the CONT_READ bit in

the ADC_CONTROL register is set to 0).

The memory map CRC function is enabled by setting the MM_

CRC_ERR_EN bit in the ERROR_EN register to 1. If an error

occurs, the MM_CRC_ERR bit in the error register is set to 1.

ROM CHECKSUM PROTECTION

On power-up, all registers are set to default values. These default

values are held in read-only memory (ROM). For added robustness,

a CRC calculation is performed on the ROM contents as well. The

CRC is performed on power-up.

The ROM CRC function is enabled by setting the ROM_CRC_

ERR_EN bit in the ERROR_EN register to 1. If an error occurs, the

ROM_CRC_ERR bit in the error register is set to 1.

When this function is enabled, the internal controller clock, if ena-

bled, remains active in the standby mode.

CRC Calculation

The checksum, which is 8 bits wide, is generated using the polyno-

mial

x8 + x2 + x + 1

To generate the checksum, the data is left shifted by eight bits to

create a number ending in eight Logic 0s. The polynomial is aligned

so that its MSB is adjacent to the leftmost Logic 1 of the data. An

XOR (exclusive OR) function is applied to the data to produce a

new, shorter number. The polynomial is again aligned so that its

MSB is adjacent to the leftmost Logic 1 of the new result, and the

procedure is repeated. This process is repeated until the original

data is reduced to a value less than the polynomial. This is the 8-bit

checksum.

Example of a Polynomial CRC Calculation (24-

Bit Word): 0x654321 (8-Bit Command and 16-

Bit Data)

An example of generating the 8-bit checksum using the polynomial

based checksum is as follows:

Initial value

011001010100001100100001

01100101010000110010000100000000

left shifted eight bits

x8 + x2 + x + 1 =

100000111

polynomial

100100100000110010000100000000

XOR result

100000111

polynomial

100011000110010000100000000

XOR result

100000111

polynomial

11111110010000100000000

XOR result

100000111

polynomial value

1111101110000100000000

XOR result

100000111

polynomial value

111100000000100000000

XOR result

100000111

polynomial value

11100111000100000000

XOR result

100000111

polynomial value

1100100100100000000

XOR result

100000111

polynomial value

100101010100000000

XOR result

100000111

polynomial value

101101100000000

XOR result

100000111

polynomial value

1101011000000

XOR result

100000111

polynomial value

101010110000

XOR result

100000111

polynomial value

1010001000

XOR result

100000111

polynomial value

10000110

checksum = 0x86

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