MAX17823H datasheet(49/129 Pages) MAXIM

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UART RX Modes

During the first preamble received after a reset, the

receiver automatically detects if the received signal is

single-ended and if so, places the receiver in single-ended

mode; therefore, the device must be reset for any change

in the RX-mode hardware configuration to be detected.

The receiver mode is indicated by the ALRTCOMMSEL

bit (for lower port) and ALRTCOMMSEU bit (for upper

port) of the FMEA1 register, as shown in Table 19. If the

RXP input is open circuit, the RX-mode detection places

the UART in single-ended mode so the port can still oper-

ate, albeit with reduced noise immunity. The host can

diagnose this condition by checking ALRTCOMMSEL and

ALRTCOMMSEU after any POR event. Any other faults

result in communication errors.

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 applications

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-chained 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 receiver.

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 loca-

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

loopback mode on the first device, checking communica-

tion, 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 and 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.

Table 19. UART RX Modes

RXP

RXN

ALRTCOMMSEn

RX MODE

Connected to data

Connected to inverse data

Differential mode (normal)

Grounded

Connected to inverse data

Single-ended mode (normal)

Open circuit (fault)

Connected to inverse data

Single-ended mode (low noise immunity)

Connected to data

Open circuit (fault)

Differential mode (communication errors)

MAX17823H

12-Channel, High-Voltage

Data-Acquisition Systems

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