MAX17841BGUE datasheet(15/33 Pages) MAXIM

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Transmit Buffer

The transmit buffer memory map is shown below. It con-

sists of four fixed-length queues, which the host uses to

store outgoing messages. At any time, one of the queues

is designated as the load queue (the queue being loaded)

and one of the queues is designated as the transmit

queue (the queue being unloaded). The load queue is

selected by the two-bit register LD_Q and the transmit

queue is selected by the two-bit register TX_Q. Each

queue consists of seven bytes.

Transmit Buffer Queues

In each queue, location 0 is reserved for the message

length and the remaining six locations are for specific

message data. The default state of each queue is as

shown in Table 7.

Clearing the Transmit Buffer

During UART initialization, it is recommended that the

host reset the transmit buffer by issuing the CLR_TX_BUF

SPI transaction (20h). This resets the transmit buffer as follows:

● TX_Q[1:0]=00b

● LD_Q[1:0]=00b

● Data in transmit buffer (28 bytes) is reset to default

state per Table 7.

Message Length

Before composing any message, the host should

compute the message’s length (in bytes, not characters)

based on both the type of command (read or write) and

the device count. The message length should include any

required fill bytes (but not preamble and stop characters).

The host writes the message length into location 0 of the

load queue, but if the specified message length is greater

than 62d, only 62d (3Eh) is actually written. If the TX_

Unlimited = 1, then the maximum message length written

is increased to 255d (FFh), but the host must service the

receive buffer accordingly to avoid any possible overflow.

If the specified message length is greater than 6 bytes, the

UART automatically appends alternating fill bytes (D3h,

C2h) as required by the battery management UART proto-

col during the latter portion of the message transmission.

Figure 11. Transmit Buffer Memory Map

Table 7. Queue Memory Map

LOCATION

DESCRIPTION

DEFAULT

VALUE

MAXIMUM DEFAULT PERMITTED

TX_UNLIMITED = 0

TX_UNLIMITED = 1

Message length

00h

3Eh

FFh

Databytesand/orfill

bytes

D3h

FFh

C2h

D3h

C2h

D3h

C2h

Tx BUFFER MEMORY MAP

0 0 1 0 1

QUEUE 0

TRANSMIT QUEUE

DATA TO UART (Tx)

8 BITS

BYTE

000

110

7 BYTES

QUEUE 1

(LD COMPLETE

Tx PENDING)

7 BYTES

QUEUE 2

(LOAD QUEUE)

DATA TO/FROM HOST

7 BYTES

QUEUE 3

(EMPTY)

7 BYTES

TX_POINTER

UART CONTROLS

INCREMENT

LD_POINTER

HOST CONTROLS

INCREMENT THROUGH SPI

TX_Q [1:0]

READ ONLY

LOCATION [2:0]

INTERNAL

1 0 0 1 0

LD_Q [1:0]

READ ONLY

LOCATION [2:0]

INTERNAL

QUEUE 0 IS THE TRANSMIT QUEUE IN THIS EXAMPLE

QUEUE 2 IS THE LOAD QUEUE IN THIS EXAMPLE

TRANSMIT BUFFER ADDRESSING

Maxim Integrated │ 15

MAX17841B

Automotive SPI Communication Interface (ASCI)

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