ADBMS6821 datasheet(24/28 Pages) AD

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Data Sheet

ADBMS6821/ADBMS6822

THEORY OF OPERATION

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Rev. B | 24 of 28

LPCM HEARTBEAT MESSAGES

The LPCM feature uses messaging between the battery monitor

devices and the timeout monitor to communicate the monitoring

status. The heartbeat message contains device count information

about the number of devices reporting passing conditions, as well

as a field of flags that indicate the types of failing conditions that

can be detected. It is sent as a 64-bit message containing a

command with a packet error check (PEC) code and data with its

own PEC code. The usage of the PEC values protects against

communication faults.

The first 32 bits contain a command and PEC, which identify

the communication as a heartbeat. Any other value is ignored by

the timeout monitor without being recognized as either a pass

heartbeat or as a fail heartbeat. A pass or fail heartbeat can only be

recognized if the first 32 bits match exactly.

The subsequent 32 bits of the message represents 16 bits of the

device count and flag data and 16 bits of PEC code. If a heartbeat

command has been recognized, this 32-bit data portion is detected

as either a pass or as a fail message. A single 32-bit value is

recognized as a pass message. For any other 32-bit value, or if

the total communication length is not exactly 64 bits long, then the

timeout monitor recognizes this as a fail heartbeat and immediately

asserts the INTR pin high without waiting for the timeout value to

elapse.

Table 24 and Table 25 show the contents of a pass heartbeat

message. Note that, unlike other communication in a BMS daisy

chain, the heartbeat message is initiated by the battery monitors

instead of being initiated by the microcontroller.

Table 24. Heartbeat Command and PEC Value

Byte1

Value

HBC0

0x00

HBC1

0x43

HBC2

0x47

HBC3

0xB2

1 HBC0 and HBC1 represent the heartbeat command. HBC2 and HBC3 repre-

sent the PEC.

Table 25. Heartbeat Data and PEC Value

Byte1

Value

HBD0

0x42

HBD1

0x00

HBD2

0x03

HBD3

0x94

1 HBD0 and HBD1 represent the heartbeat data. HBD2 and HBD3 represent the

PEC.

LPCM EXPANDED STATE DIAGRAM

Figure 42 shows the expanded state diagram for the ADBMS6821/

ADBMS6822 transceivers when they are set up to work as an

LPCM timeout monitor, that is, when configured as a peripheral

(MSTR connected to GND) with the LPCM timeout monitor function

enabled (XCVRMD pin has 20 kΩ to GND). When compared to the

isoSPI state diagram shown in Figure 38, Figure 42 includes an

extra state named listen.

When in the listen state, the transceiver works as an LPCM timeout

monitor. Note that the time it takes for the transceiver to enter the

listen state from the ready state when there is no isoSPI activity is

tLISTEN. tLISTEN reduces the average power consumption during the

LPCM timeout monitor operation.

Figure 42. LPCM Expanded State Diagram

LPCM SUPPLY CURRENT CONSUMPTION

When the ADBMS6821/ADBMS6822 transceivers are configured

as an LPCM timeout monitor, they stay in the listen state for most

of the time. When a transceiver receives the heartbeat message,

it enters the ready state and the isoSPI receiver wakes. After the

heartbeat message is received, the transceiver goes back to the lis-

ten state after tLISTEN. The average power consumption, therefore,

depends on the heartbeat period that the BMS stack monitors use

during the LPCM operation.

When the transceiver enters the ready state, the supply current

increases to a value of IVDD/VP(RDY_LPCM). Note that this is lower

than the ready state current in regular non LPCM operation.

The instantaneous current consumption for a chosen heartbeat

period is broken up as follows:

►

IVDD/VP(RDY_LPCM) for the duration of the heartbeat message

(which, in a typical application, is ~700 µs) and the tLISTEN time

(typically ≈ 250 μs). The total time is approximately 1 ms.

►

IVP(LSTN) for the rest of the heartbeat period.

In a typical application, when the transceiver is configured as an

LPCM timeout monitor, VDD is expected to be 0 V during the

timeout monitor operation, and the device is powered only using the

battery power supplied to the VP pin. The average supply current

consumption for a timeout period is therefore as follows:

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