MAX17843 datasheet(98/131 Pages) MAXIM

98 / 131 page

The host uses the algorithm to process all bytes received

in the command packet prior to the PEC byte itself.

Neither the PEC nor the alive-counter bytes are part of the

calculation. The bits are processed in the order they are

received, LSB first. A byte-wise pseudo-code algorithm is

shown in Figure 71, but lookup table solutions are also

possible to reduce host calculation time.

For commonly issued command packets, the host can

pre-calculate (hard-code) the PEC byte. For commonly

used partial packets, the CRC value of a partial calcula-

tion can be used as the initial value for a subsequent

run-time calculation.

Figure 71. PEC Calculation Pseudocode

Function PEC_Calculation(ByteList(), NumberOfBytes, CRCByte)

{

// CRCByte is initialized to 0 for each ByteList in this implementation, where

// ByteList contains all bytes of a single command. It is passed into the

// function in case a partial ByteList calculation is needed.

// Data is transmitted and calculated in LSb first format

// Polynomial = x^8+x^6+x^3+x^2+1

POLY = &HB2 // 10110010b for LSb first

//Loop once for each byte in the ByteList

For ByteCounter = 0 to (NumberOfBytes – 1)

(

//Bitwise XOR the current CRC value with the ByteList byte

CRCByte = CRCByte XOR ByteList(Counter1)

//Process each of the 8 CRCByte remainder bits

For BitCounter = 1 To 8

(

// The LSb should be shifted toward the highest order polynomial

// coefficient. This is a right shift for data stored LSb to the right

// and POLY having high order coefficients stored to the right.

// Determine if LSb = 1 prior to right shift

If (CRCByte AND &H01) = 1 Then

// When LSb = 1, right shift and XOR CRCByte value with 8 LSbs

// of the polynomial coefficient constant. “/ 2” must be a true right

// shift in the target CPU to avoid rounding problems.

CRCByte = ((CRCByte / 2) XOR POLY)

Else

//When LSb = 0, right shift by 1 bit. “/ 2” must be a true right

// shift in the target CPU to avoid rounding problems.

CRCByte = (CRCByte / 2)

End If

//Truncate the CRC value to 8 bits if necessary

CRCByte = CRCByte AND &HFF

//Proceed to the next bit

Next BitCounter

)

//Operate on the next data byte in the ByteList

Next ByteCounter

)

// All calculations done; CRCByte value is the CRC byte for ByteList() and

// the initial CRCByte value

Return CRCByte

}

MAX17843

12-Channel, High-Voltage Smart Sensor

Data-Acquisition Interface

www.maximintegrated.com

Maxim Integrated │ 98

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