MAX17843 datasheet(58/131 Pages) MAXIM

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Data-Check Byte

The host uses the returned data-check byte to promptly

determine if any communication errors occurred during

the packet transmission and to check if alert flags are set

in any devices, as shown in Table 26. The data-check

byte is returned by the READALL and READDEVICE

commands. For READDEVICE, the data-check byte is

updated only by the addressed device.

The data-check byte sent by the host is a seed value nor-

mally set to 00hN although nonzero values can be used

as a diagnostic. Each device logically ORs the received

data-check byte with its own status and transmits it to the

next device. A PEC error detected by any device will set

the ALRTPEC bit in the STATUS register and, by exten-

sion, the ALRTPEC and ALRTSTATUS bits in the data-

check byte.

PEC Byte

The PEC byte is a CRC-8 packet-error check sent by

the host with all read and write commands. If any device

receives an invalid PEC byte, it sets the ALRTPEC bit in

the STATUS register. During any write transaction, a device

does not execute the write command internally unless the

received PEC matches the expected calculated value. For

read commands, the device must return its own calculated

PEC byte based on the returned data. The host should

verify that the received PEC byte matches the calculated

value and if an error is indicated, the data should be dis-

carded. See Applications Information section for details on

the PEC calculation.

Alive-Counter Byte

The alive-counter byte is the last data byte of the com-

mand packets (except HELLOALL) if the ALIVECNTEN

bit is set in the DEVCFG1 register. The host typically

transmits the alive-counter seed value as 00h, but any

value is permitted. For WRITEALL or READALL com-

mands, each device retransmits the alive counter, incre-

mented by one. For WRITEDEVICE or READDEVICE

commands, only the addressed device will increment it.

The alive counter is not used in the HELLOALL command.

If the alive counter reaches FFh, the next device incre-

ments it to 00h.

Since the alive counter comes after the PEC byte, an

incorrect PEC value does not affect the incrementing of

the alive-counter byte. Also, the PEC calculation does

not include the alive-counter byte. The host should verify

that the alive counter equals the original seed value + the

number of devices, considering that if the alive-counter

reaches FFh, the next device increments it to 00h.

Fill Bytes

In the READALL command, the host sends two fill bytes

for each device in the daisy-chain. The fill bytes are the

locations within the packet and are used by the device to

place the read data. The fill-byte values transmitted by the

MAX17841B interface alternate between C2h and D3h.

As the command packet propagates through the device,

the device overwrites the appropriate fill bytes with the

determine which specific fill bytes in the packet are to be

overwritten.

For a READDEVICE command, only two fill bytes are

required since only one device responds (returning two

data bytes). Also, fill bytes are not required for write com-

mands because the data received is exactly the same as

the data retransmitted.

Table 26. Data-Check Byte

BIT

NAME

DESCRIPTION

ALRTPEC

ALRTPEC is set.

ALRTFMEA

ALRTFMEA1 or ALRTFMEA2 is set.

ALRTSTATUS

STATUS bit other than ALRTFMEA1, ALRTFMEA2, ALRTOV, and ALRTUV is set.

CHECK

Check bit. Value received is forwarded.

CHECK

Check bit. Value received is forwarded.

ALRTOV

ALRTOV is set.

ALRTUV

ALRTUV is set.

CHECK

Check bit. Value received is forwarded.

MAX17843

12-Channel, High-Voltage Smart Sensor

Data-Acquisition Interface

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