MAX17841BGUEV datasheet(16/33 Pages) MAXIM

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Writing the Load Queue

Amessage,notincludingfillbytes,consistsofthree(3)

tosix(6)bytes.TheHELLOALLsequence,forexample,

isthreebytes:57h,00h,00h(firstaddresssettozero).

Since no fill bytes are required, the total message length

is3bytes.Therefore,thehostshouldwritetheloadqueue

with the following data in Table 8.

The host can write the load queue starting at any location

within the queue by using appropriate SPI commands

listed in SPI transaction Table 9. However, if the host

attempts to write beyond location 6 of the queue, the

additional data is ignored.

The UART never attempts to transmit the queue selected

by LD_Q because the host may be in the process of

loading it or, even if it has finished loading, may need to

verify (read) the contents of the load queue by using the

RD_LD_Q transaction (C1h). The host can then select

the next queue in sequence for loading by performing the

WR_NXT_LD_Qtransaction(B0h),whichincrementsthe

LD_Qvalue.Itisonlywhenthisincrementoccursthatthe

UART starts transmitting the data in the previously loaded

queue.ForbothLD_Q[1:0]andTX_Q[1:0],valuesof3h

increment to 0h.

Filling the Transmit Buffer

ThehostcanloadallavailablequeuesuntilLD_Q=TX_Q

-1.Inthisstate,thetransmitbufferisfull(TX_Full_Status

true). In this condition, the host cannot start loading the

transmit queue because the UART may still be unloading/

transmitting data. If the transmit buffer is full and the host

attempts to perform a WR_NXT_LD_Q transaction and

thus attempts to load the transmit queue, the increment

does not occur and an overflow condition is indicated

(TX_Overflow_Status true). The only time the host can

write the transmit queue is when the transmit buffer is

empty (TX_Q = LD_Q), which is the default state. This

state can also occur when the UART finishes sending the

last loaded message and thus creates an empty transmit

buffer.

Message Transmission

Whenever LD_Q =TX_Q, the transmit buffer is consid-

ered empty (TX_Empty_Status is true) because either

the host has not yet finished loading the selected queue

or the host has written but not yet verified the queue.

However,oncethehostisfinishedservicingthequeue,it

performsaWR_NXT_LD_Qtransactiontoselectthenext

queue.Oncethisoccurs,thetransmitbufferisnolonger

consideredemptybecauseLD_Q≠TX_Q.

The UART unloads/transmits the transmit queue if the fol-

lowing conditions are met:

● TheUARTisinTransmit_Queuemode(TX_Queuebit

is set)

● The transmit buffer has at least one loaded queue

(TX_Empty_Statusisfalse)

● Thereissufficientspaceinthereceivebufferforthe

message(RX_Space_≥MessageLength)

Note:Thelimitationonavailablespaceinthereceivebuf-

fercanberemovedbysettingtheTX_Unlimitedbit.

Once the transmit conditions are met, the UART auto-

matically starts unloading the transmit queue until the

entire message, including any required fill bytes, has

been transmitted. After the transmission is complete, the

contents of the transmit queue are reset to their default

values and the queue is once again available to the host

for loading.

Receive Buffer

The receive buffer is a 62-byte circular buffer that the

host can read with the SPI, but can only be loaded by

theUARTasitreceivesdata.Itutilizesthreepointersas

shown in the receive buffer memory map (Figure 12).

● RX_RD_POINTER:Readpointerorbufferlocationto

be read by host (default 00h, read-only)

● RX_WR_POINTER:Writepointerorbufferlocationto

be written by UART (default 01h, read-only)

● RX_NXT_MSG_POINTER:Bufferlocationthatisstart

of next unread message (default 00h, read-only)

In the default state, where the read pointer is one less

than the write pointer, the receive buffer is considered

empty(RX_Empty_Statusistrue).Anyreceivebufferdata

readinthisconditionwillbezero.

Table 8. Example of Queue Loaded with

Message HELLOALL

LOCATION

VALUE

DESCRIPTION

03h

Message length

57h

Command byte

00h

Address byte

00h

Databyte

C2h

Notwritten

D3h

Notwritten

C2h

Notwritten

Maxim Integrated │ 16

MAX17841B

Automotive SPI Communication Interface (ASCI)

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