MAX17841BGUE datasheet(17/33 Pages) MAXIM

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Clearing the Receive Buffer

DuringUARTinitialization,itisrecommendedthatthehost

clearthereceivebufferbyissuingtheCLR_RX_BUFSPI

transaction(E0h).Thisresetsthereceivebufferasfollows:

● RX_RD_Pointer:00h

● RX_WR_Pointer:01h

● RX_NXT_MSG_POINTER:00h

● Datainreceivebuffer(62bytes)isclearedto00h

If the receive buffer is cleared during Transmit Preambles

mode, the state of the buffer cannot be guaranteed.

Therefore, after disabling the Transmit Preambles mode,

the host should wait until all transmitted preambles have

been received before clearing the buffer. Since the first

keep-alive stop character received after the last preamble

results in a null message, the host can simply wait until

the buffer is no longer empty (RX_Empty_Status = 0)

before clearing the buffer.

The RX Clear Buffer command acts an an asynchronous

reset not only for the RX Buffer, but also for the UART

receiver logic. When the UART receiver logic is reset, it

must resynchronize to the incoming UART signal before

bytes can be properly processed. This is accomplished

by receiving either a preamble byte or an idle state

lasting at least one UART byte period. The preamble used to

resynchronize the data stream should not be the same

preamble that is at the beginning of the next transmitted

message. The application should make sure that one of

these conditions is met following an RX buffer clear prior to

sending the next message from the MAX17841B.

Receiving Messages

UART messages are framed with the preamble and stop

characters. If the UART receiver decodes a valid pream-

ble, it prepares to receive a message but it does not store

thepreambleinthereceivebuffer.Oncedataisreceived,

thebufferisnolongerempty(RX_Empty_Status=0)and

the UART sequentially stores decoded data bytes in the

receive buffer until either a stop character or another pre-

amble is received. When the stop character is received at

the end of a message, the UART stores it in the receive

bufferasanullbyte(00h)andsetstheRX_Stop_Status

bit. The RX_Stop_Status bit is subsequently cleared

when all unread messages have been read (buffer empty)

or the next preamble is detected. The host can set the

RX_Stop_INT_Enablebitandmonitortheinterruptlineto

determine when to service the receive buffer.

When the host services the receive buffer, three bits in the

RX_Byte register indicate specific information about the

bytebeingread(thebyteaddressedbyRX_RD_Pointer),

which is useful for error checking:

● First_Bytebit:Indicatesthatthebyteisthefirstdata

byte in a message (the corresponding character was

preceded by preamble character).

● Byte_Error bit: Indicates that the byte may contain

an error (the corresponding character contained a

Manchester and/or parity error). This bit drives the

RX_Errorinterrupt.

● Last_Bytebit:Indicatesthatthebyteisthelastbytein

a message (the corresponding character was a stop

character and was stored as a null byte).

Message Exceptions

If a message is not framed with a valid preamble, then the

UART ignores the data and does not store it.

If a message is not framed with a stop character, then

the preamble of the next message serves to delineate

betweenthetwomessages.However,thefirstmessage

has no stop character stored.

If the UART receives a preamble followed by a stop

character it stores a null message in the receive buffer

consisting of a single null byte (00h). This occurs when

a keep-alive stop character is received after Transmit

Preambles mode is disabled. In this use case, the receive

Figure 12. Receive Buffer Memory Map

RECEIVE BUFFER MEMORY MAP

EACH MESSAGE IS

VARIABLE LENGTH

AVAILABLE

(EMPTY)

FIRST BYTE OF MESSAGE 2

DATA TO HOST

RX_RD_POINTER

RX_NXT_MESSAGE

RX_WR_POINTER

FIRST BYTE OF MESSAGE 3

DATA FROM UART

AVAILABLE

(EMPTY)

1 1 1 1 1 0

0 0 0 0 0 0

MESSAGE 1

(ALREADY READ)

8 BITS

MESSAGE 2

(BEING READ)

PART OF

MESSAGE 3

(BEING LOADED

BY UART)

Maxim Integrated │ 17

MAX17841B

Automotive SPI Communication Interface (ASCI)

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