ATSHA204A datasheet(44/93 Pages) MICROCHIP

44 / 93 page

sequence, the device retains the contents of the two SRAM registers (for example TempKey and RNG

seed).

Normally, all command sequences must complete within tWATCHDOG if they require a state that is stored in

the SRAM registers. The system software can use this idle mode mechanism between commands to

implement a longer command sequence than can be completed during a single watchdog interval.

8.5

Command Sequence

8.5.1

Command Packets

The command packet is broken down as shown in the following table:

Table 8-3. Command Packets

Byte # Name

Meaning

Command

Command Flag (see Word Address Values for the I2C mode of operation and I/O

Flags for Single Wire Interface mode). Not included in Count or CRC field.

Count

Packet size: Includes Count, Opcode, Param1, Param2, Data and CRC. Does not

include Command Flag.

Opcode

ATSHA204A operation being called.

Param1

First Parameter. One byte always present.

4 – 5

Param2

Second Parameter. Two bytes always present.

Data

Optional Data based on Command being called.

N-1to N Checksum

CRC-16. The CRC polynomial is 0x8005. Includes Count, Opcode, Param1,

Param2 and Data. Does not include Command Flag.

After the ATSHA204A receives all the bytes in a block, the device transitions to the busy state and

attempts to execute the command. Neither status nor results can be read from the device when it is busy.

During this time, the device’s I/O interface ignores all SDA transitions regardless of the I/O interface

selected. The command execution delays are listed in Section Read Operations within the Data Zone.

If an insufficient number of bytes are sent to the device when it is in one-wire mode, the device

automatically transitions to the low-power sleep state after the tTIMEOUT interval. In I2C mode, the device

continues to wait for the remaining bytes until the watchdog timer limit, tWATCHDOG, is reached or a Start/

Stop condition is received by the device.

In the individual command descriptions in Table 8-8 through Table 8-41, the size column describes the

number of bytes in the parameter documented in each particular row. If the input block size for a

particular command is incorrect, then the device responds differently depending upon the command. It

may not return an error indication in all circumstances (see Section Status/Error Codes).

8.5.2

Command Opcodes, Short Descriptions and Execution Times

During parsing of the parameters and the subsequent execution of a properly received command, the

device is busy and can not respond to transitions on the pins. The interval during which the device is busy

varies depending upon the command and its parameter values, the state of the device, the environmental

conditions and other factors according to the following table.

ATSHA204A

Security Commands

DS40002025A-page 44

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