MAX17841BGUE datasheet(19/33 Pages) MAXIM

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should be checked again for data errors (e.g., parity errors)

prior to initiating transmission of the next message.

If multiple messages are received without being read,

thenanoverflowcanoccurandtheUARTsetstheRX_

Overflow_Status bit.This occurs when the write pointer

has incremented until it is one less than the read pointer,

at which point the UART no longer increments it. In this

case, the last data byte is overwritten.

Reading Messages

The host can use two different SPI transactions to read

the receive buffer:

● RD_RX_BUF(91h):Startsreadingatthecurrentread

pointer location

● RD_NXT_MSG(93h):Startsreadingatthestartofthe

next unread message

Duringanyreadtransaction,thehostmaycontinueread-

ing data until the end of the message, after which the data

read will be 00h. The host cannot continue reading into

the next message, if there is one.

During any read transaction, the UART increments the

read pointer after the data byte is read so that if the SPI

transaction is prematurely terminated in the middle of

the byte, then the same location is resent on the next

RD_MSGSPItransaction.Thisallowsthehosttostopa

readandrestartitwithoutlosingdata.Eachbyteinthe

buffer is cleared after it is read and is eventually available

to the UART for storing incoming data.

Applications Information

Transaction Sequence for UART Initialization

In the example shown in Table 10,thehostinitializescom-

munication with two UART slave devices. SHDN must be

deassertedfirst.TransactionstopollRX_STATUSregis-

ter are repeated until the poll is successful or times out.

It is recommended that all writes to configuration registers

be verified by reading back the register data. Transmit

buffer data can be verified by reading the buffer contents

or by reading the transmitted data in the receive buffer.

Transaction Sequence for UART Write and

Read

In the example shown in Table 11, the host communicates

with two UART slave devices to:

● WritethevalueB2B1htothedeviceregisteraddress

0x12 for all slave devices using a WRITEALL com-

mand sequence

● ReadbackthevalueB2B1hfromthedeviceregister

address 0x12 for all slave devices using READALL

command sequence.

This example assumes that the slave devices have been

configured with the alive counter enabled. To execute

these two command sequences, the host performs the

SPI transactions listed in Table 11.

Table 10. UART Daisy-Chain Initialization Sequence

DIN

DOUT

DESCRIPTION

TRANSACTION 1

EnableKeep-Alivemode(priortotheUARTslavewake-uptopreventshutdown)

10h

xxh

WriteConfiguration3register

05h

xxh

Set keep-alive period to 160µs

TRANSACTION 2

EnableRxInterruptflagsforRX_ErrorandRX_Overflow

04h

xxh

WriteRX_Interrupt_Enableregister

88h

xxh

SettheRX_Error_INT_EnableandRX_Overflow_INT_Enablebits

TRANSACTION 3

Clearreceivebuffer

E0h

xxh

Clearreceivebuffer

TRANSACTION 4

Wake-up UART slave devices (transmit preambles)

0Eh

xxh

WriteConfiguration2register

30h

xxh

EnableTransmitPreamblesmode

TRANSACTION 5

WaitforallUARTslavedevicestowakeup(pollRX_Busy_Statusbit)

01h

xxh

ReadRX_Statusregister(RX_Busy_StatusandRX_Empty_Statusshouldbetrue)

xxh

21h

IfRX_Status=21h,continue.Otherwise,repeattransactionuntiltrueortimeout.

TRANSACTION 6

EndofUARTslavedevicewake-upperiod

0Eh

xxh

WriteConfiguration2register

10h

xxh

DisableTransmitPreamblesmode

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MAX17841B

Automotive SPI Communication Interface (ASCI)

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