AN4286 datasheet(9/38 Pages) STMICROELECTRONICS

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Bootloader command set

AN4286

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DocID024458 Rev 2

2.1

Safety of communication

All communication from the programming master to the slave is verified in the following way.

•

Checksum: received blocks of data bytes are XORed. A byte containing the computed

XOR of all previous bytes is added to the end of each communication (checksum byte).

By XORing all received bytes, data + checksum, the result at the end of the packet

must be 0x00.

•

If the received data is one byte, then its checksum is the bit negation of the value (the

checksum of 0x02 is 0xFD).

•

For each command, the master sends three bytes: a Start Of Frame (SOF = 0x5A), a

byte representing the command value and its complement (XOR of the command and

its complement = 0x00).

•

Each packet is either accepted (ACK answer) or discarded (NACK answer).

–

ACK = 0x79

–

NACK = 0x1F

Note:

The master’s frame can be one of the following.

Send command frame: the master initiates communication as master transmitter and sends

two bytes to the slave: command code + Xor.

Wait for ACK/NACK frame: the master initiates an SPI communication as master receiver

and receives one byte from the slave: ACK or NACK.

Receive Data frame: the master initiates an SPI communication as master receiver and

receives the response from the slave. The number of received bytes depends on the

command.

Send Data frame: the master initiates an SPI communication as master transmitter and

sends the needed bytes to the slave. The number of transmitted bytes depends on the

command.

2.2

Get command

The Get command enables you to get the version of the bootloader and the supported

commands. When the bootloader receives the Get command, it transmits the bootloader

version and the supported command codes to the master, as described in Figure 6.

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