ATSHA204A datasheet(22/93 Pages) MICROCHIP

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Single-Wire Interface

In the Single-Wire Interface mode, communications to and from the ATSHA204A take place over the SDA

pin, a single, asynchronously timed wire and the SCL pin is ignored.

The sleep current specification values are guaranteed only if the SCL pin is held low or left unconnected.

The overall communications structure is a hierarchy: The table below shows the tokens used for the

Single-Wire Interface with a standard RS-232 port. The Host UART port should be set to 7-bit data words

and 230.4 kBaud data rate.

Table 5-1. Wake and I/O Tokens

Token Type

Token

Value

Start (1)

Wake Token LSb: MSb

Stop (1)

Wake (2)

0x00

Logic 0 (3)

0x7D

Logic 1 (3)

0X7F

Note:

All Tokens must be preceded by a LOW Start Pulse to synchronize the data capture and end with a

HIGH Stop value.

A Wake Token creates a low pulse great enough to wake up the device.

Logic 0, Logic 1 I/O tokens represent a single bit of data. 8 I/O tokens would be needed to create a

single byte of data.

I/O Flags - Flags consist of eight tokens (bits) that convey the direction and meaning of the next group of

bits (if any) that may be transmitted. Flags are always transmitted LSb first.

Blocks - Blocks of data follow the command and transmit flags. They incorporate both a byte count and a

checksum to ensure proper data transmission.

Packets - Packets of bytes form the core of the block (minus the byte count and CRC). They are either

the input or output parameters of a CryptoAuthentication command or status information from the

ATSHA204A.

5.1

I/O Tokens

There are a number of I/O tokens that may be transmitted over the Single-Wire Interface:

•

Input (to the ATSHA204A)

–

Wake: wake the device up from either sleep or idle states.

–

Zero: send a single bit from the system to the device with a value of zero.

–

One: send a single bit from the system to the device with a value of one.

•

Output (from the ATSHA204A)

–

ZeroOut: send a single bit from the device to the system with a value of zero.

–

OneOut: send a single bit from the device to the system with a value of one.

The waveforms are the same in either direction. There are some differences in timing; however, based on

the expectation that the Host has a very accurate and consistent clock, while the ATSHA204A has

ATSHA204A

Single-Wire Interface

DS40002025A-page 22

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