MAX17843 datasheet(53/131 Pages) MAXIM

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UART Loopback

For the last device in the stack, the data must be looped

back from the upper transmitter to the upper receiver. This

is known as loopback and can be configured externally

(default) or internally.

External Loopback Mode

External loopback mode (default) uses a two-wire cable to

connect the upper transmitter (TXU) to the upper receiver

(RXU). The external loopback has two advantages:

1) It is quicker to determine device count for applica-

tions where the host does not assume what the

device count is.

2) It helps to match the supply current of the last device

to that of the other daisy-chain devices (because the

hardware configuration is identical).

Internal Loopback Mode

Internal-loopback mode (LASTLOOP = 1) routes the

upper-port transmit data internally to the upper-port receiv-

er. Any signal present on the upper-port receiver input

pins is ignored in the internal loopback mode; therefore,

when LASTLOOP is set, the write command that was

forwarded to any upstack devices is interrupted in the

downstack direction. The host should expect this and read

the LASTLOOP bit to verify that the write was successful. If

the MAX17841B interface is used, its receive buffer should

be cleared before changing LASTLOOP, and cleared again

after changing the loopback configuration because the

communication was interrupted.

Internal-loopback mode is useful to diagnose the location

of a daisy-chain signal break by enabling the internal-loop-

back mode on the first device, checking communication,

then moving the loopback mode to the next device, and

continuing up the stack until communication is lost.

Baud Rate Detection

The UART can operate at a baud rate of 2Mb/s (default),

1Mb/s, or 0.5Mb/s. The baud rate is controlled by the host

and is automatically detected by the device when the first

preamble character is received after reset. If the host

changes the baud rate after reset, it must issue another

reset, which can be done by setting the SPOR bit, and

resend a minimum of 2 x n preambles (where n is the

number of devices). The 2 x n preambles are necessary

since the transmitter for the upper port will not transmit

data until the lower-port receiver has detected the baud

rate; likewise, the transmitter on the lower port will not

transmit data until the upper-port receiver has detected

the baud rate. A simple way to do this is for the host to

start transmitting preambles and stop when a preamble

has been received back at the host RX port.

TX Adaptive Mode for Single-Ended Mode

To overcome the error tolerance limitation when connect-

ing a MAX17843 to a conventional UART port, an adaptive

transmit-timing feature has been added. The feature works

by monitoring the location of the incoming Manchester

transitions at the RXL port, with respect to the local clock,

to calculate a correction factor. This correction factor is

then applied to the TXL port so the outgoing downstack

signal has similar timing characteristics to the incoming

upstack signal. With this adaptive transmit timing, the

interface between a conventional UART node and a Maxim

proprietary battery-monitoring system node has a tolerance

for baud-rate mismatch that is much higher than that of

the conventional receiver port alone giving a high level of

timing margin for direct connection applications. TX adap-

tive mode should be enabled only on the bottom device

(device connected to BMS micro or Maxim SPI-to-UART

bridge IC).

Battery-Management UART Protocol

The battery-management UART protocol uses the follow-

ing features to maximize the integrity of the communica-

tions:

● All transmitted data bytes are Manchester-encoded,

where each data bit is transmitted twice with the 2nd

bit inverted (G.E. Thomas convention).

● Every transmitted character contains 12 bits that

include a start bit, a parity bit, and two stop bits.

● Read/write packets contain a CRC-8 packet error

checking (PEC) byte

● Each packet is framed by a preamble character and

STOP character.

● Read packets contain a data-check byte for verifying

the integrity of the transmission.

The protocol is also designed to minimize power-

”consumption by allowing slave devices to shut down if

the UART is idle for a specified period of time. The host

must periodically transmit data to prevent shutdown,

unless the SHDNL input is driven externally.

MAX17843

12-Channel, High-Voltage Smart Sensor

Data-Acquisition Interface

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