AN4221 datasheet(7/51 Pages) STMICROELECTRONICS

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AN4221

Bootloader command set

No-Stretch commands

No-Stretch commands allows executing Write, Erase, Write Protect, Write Unprotect, Read

Protect and Read Unprotect operations without stretching I2C line while bootloader is

performing the operation. These commands allows communicating with other devices on

the bus while bootloader performs operation that require waiting time.

The difference between these commands and the standard commands is at the end of the

command: When hosts requests ACK/NACK at the end of the command, instead of

stretching the I2C line, the bootloader responds with a third state which is Busy (0x76).

When Host receives Busy state, it should poll again on the state and read one byte till it

receives ACK or NACK response.

Communication safety

All communication from the programming host to the device is verified by 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.

For each command, the host sends a byte and its complement (XOR = 0x00).

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

•

ACK = 0x79

•

NACK = 0x1F

With No-Stretch commands Busy state is sent instead of ACK or NACK when an operation

is ongoing:

•

BUSY= 0x76

Note:

The host's frame can be one of the following:

•

Send Command frame: The host initiates communication as master transmitter, and

sends two bytes to the device: command code + XOR.

•

Wait for ACK/NACK frame: The host initiates an I2C communication as master

receiver, and receives one byte from the device: ACK or NACK or BUSY.

•

Receive Data frame: The host initiates an I2C communication as master receiver, and

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

command.

•

Send Data frame: The host initiates an I2C communication as master transmitter, and

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

the command.

Caution:

For I2C communication, a timeout mechanism is implemented which must be respected for

Bootloader commands to be executed correctly. This timeout is implemented between two

I2C frames in the same command. For example, for a Write memory command, a timeout is

inserted between the command-sending frame and address memory-sending frame. Also,

the same timeout period is inserted between two successive instances of data reception or

2. Read protection - When the RDP (read protection) option is active, only this limited subset of commands is available. All

other commands are NACKed and have no effect on the device. Once the RDP has been removed, the other commands

become active.

3. Please refer to STM32 product datasheet and AN2606: STM32 microcontroller system memory boot mode to know which

memory spaces are valid for these commands.

4. No-Stretch commands are available only with I2C protocol V1.1.

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