MAX17841B datasheet(10/33 Pages) MAXIM

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Battery Management UART Protocol

The ASCI uses a UART protocol specifically designed for

Maxim battery management devices. This protocol uses

the following features to maximize the integrity of the

communications:

● All transmitted data are Manchester-encoded where

each data bit is transmitted twice with the second bit

inverted(G.E.Thomasconvention).

● Every transmitted character contains 12 bits that

includeaSTARTbit,aparitybit,andtwoSTOPbits.

● Each message contains a CRC-8 packet error-

checking(PEC)byte

● Eachmessageisframedbyapreamblecharacterand

stop character.

● Each received message contains a data-check byte

for verifying the integrity of the transmission.

The protocol is also designed to minimize power con-

sumption by allowing slave devices to shut down if the

data link is idle for a specified period of time. To prevent

the unintentional shutdown of slave devices, the host

shouldenabletheASCI’sKeep-Alivemodetoperiodically

transmit stop characters. The time period between stop

characters is configurable by the host.

UART Messages

A message is defined as a sequence of UART characters.

The message starts with a preamble character, followed

by data characters, and ending with a stop character.

Eachcharacterconsistsofthefollowing12bits:

● OneSTARTbit

● Eightdatabits(LSBfirst)

● Oneparitybit(even)

● TwoSTOPbits

Eachdatabyteistransmittedandreceivedastwosepa-

rate characters, one 12-bit character for each 4-bit data

nibble.EachManchester-encodednibbleactuallyrequires

eight data bits: four true bits and four inverted bits.

In its default configuration, when the ASCI transmits a mes-

sage, it automatically performs the following functions:

● Framesthemessagewiththerequiredpreamblechar-

acter at the beginning of the message.

● Manchesterencodeseachdatanibbleandtransmits

each encoded nibble with the required START, parity,

andSTOPbits.

● Transmitsthemessageattheconfiguredbaudrateof

0.5Mbps, 1Mbps, or 2Mbps.

● Framesthemessagewiththerequiredstopcharacter

at the end of the message.

These automatic functions can be disabled by enabling

the following special transmit modes:

● Transmit No Preamble mode (eliminates preamble

characters)

● TransmitNoStopmode(eliminatesstopcharacters)

● Transmit Raw Data mode (transmits data with no

Manchester encoding)

● Receive Raw Data mode (receives data as not

Manchester encoded)

Preamble Character

The preamble is a framing character that the UART

generates to signal the beginning of a message. It is

transmittedasanunencoded15h,butisstillaDC-balanced

character.Ifanybit(s)otherthantheSTOPbitsdeviate

from the unique preamble sequence, the character is

not interpreted as a valid preamble, but rather as a data

character.

Figure 7. UART Timing for a Preamble

OPTIONAL

IDLE

E = 1

IDLE

DISABLE

OPTIONAL

IDLE

ENABLE

Maxim Integrated │ 10

MAX17841B

Automotive SPI Communication Interface (ASCI)

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