ATSHA204A datasheet(24/93 Pages) MICROCHIP

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5.3

Synchronization

Because the communications protocol is half-duplex, there is the possibility that the system and the

ATSHA204A can fall out of synchronization with each other. In order to speed recovery, the device

implements a timeout that forces it to sleep under certain circumstances.

5.3.1

I/O Timeout

After a leading transition for any data token has been received, the ATSHA204A expects the remaining

bits of the token to be properly received by the device within the tTIMEOUT interval. Failure to send enough

bits or the transmission of an illegal token (a low pulse exceeding tZLO) causes the device to enter the

sleep state after the tTIMEOUT interval.

The same timeout applies during the transmission of the command block. After the transmission of a legal

command flag, the I/O timeout circuitry is enabled until the last expected data bit is received.

Note: The Timeout Counter is reset after every legal token and the total time to transmit the command

may exceed the tTIMEOUT interval while the time between bits may not.

The I/O timeout circuitry is disabled when the device is busy executing a command.

5.3.2

Synchronization Procedures

If the device is not busy when the system sends a transmit flag, the device should respond within

tTURNAROUND. If tEXEC time has not already passed, the device may be busy and the system should poll or

wait until the maximum tEXEC time has elapsed. If the device still does not respond to a second transmit

flag within tTURNAROUND, it may be out of synchronization. At this point, the system may take the following

steps to reestablish communication:

Wait tTIMEOUT.

Send the transmit flag.

If the device responds within tTURNAROUND, then the system may proceed with more commands.

Send a Wake token.

Wait tWHI.

Send the transmit flag.

The device should respond with a 0x11 status within tTURNAROUND, at which time system may

proceed with commands.

Any command results in the I/O buffer may be lost when the system and device lose synchronization.

5.4

Sharing the Interface

Multiple CryptoAuthentication devices may share the same interface, as follows:

System issues a Wake token (Section Watchdog Failsafe) to wake-up all devices.

The system issues the Pause command to put all but one of the devices into idle mode. Only the

remaining device then sees any commands that the system sends. When the system has

completed talking to the one active device, it sends an idle flag, which the idle devices ignore, but

puts the single remaining active device into the idle mode. See Section Pause Command for more

details.

Steps 1 and 2 are repeated for each device on the wire. If the system has completed communications

with the final device, it should wake all the devices up and then put all the devices to sleep to reduce total

power consumption.

ATSHA204A

Single-Wire Interface

DS40002025A-page 24

ATSHA204A Datasheet(PDF) 24 Page - Microchip Technology

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