ST AN3155 Installation and operating instructions

April 2010 Doc ID 17066 Rev 2 1/37

AN3155

Application note

USART protocol used

in the STM32™ bootloader

Introduction

This application note describes the USART protocol used in the STM32 microcontroller

bootloader. It details each supported command. For more information about the USART

hardware resources and requirements for your device bootloader, please refer to the

“STM32 system memory boot mode” application note (AN2606).

Related documents

Available from www.st.com:

AN2606 “STM32 system memory boot mode”

www.st.com

Contents AN3155

2/37 Doc ID 17066 Rev 2

Contents

1 USART bootloader code sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 Choosing the USARTx baud rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1 Minimum baud rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2 Maximum baud rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3 Bootloader command set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.1 Device-dependent bootloader parameters . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.2 Get command . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.3 Get Version & Read Protection Status command . . . . . . . . . . . . . . . . . . 10

3.4 Get ID command . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.5 Read Memory command . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.6 Go command . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.7 Write Memory command . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.8 Erase Memory command . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.9 Extended Erase Memory command . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.10 Write Protect command . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.11 Write Unprotect command . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.12 Readout Protect command . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.13 Readout Unprotect command . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

4 Bootloader protocol version evolution . . . . . . . . . . . . . . . . . . . . . . . . . 35

5 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

AN3155 List of tables

Doc ID 17066 Rev 2 3/37

List of tables

Table 1. USART bootloader commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Table 2. Bootloader protocol versions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Table 3. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

List of figures AN3155

4/37 Doc ID 17066 Rev 2

List of figures

Figure 1. Bootloader for STM32 with USART . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Figure 2. Get command: host side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 3. Get command: device side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 4. Get Version & Read Protection Status command: host side . . . . . . . . . . . . . . . . . . . . . . . 11

Figure 5. Get Version & Read Protection Status command: device side. . . . . . . . . . . . . . . . . . . . . . 11

Figure 6. Get ID command: host side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Figure 7. Get ID command: device side. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 8. Read Memory command: host side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Figure 9. Read Memory command: device side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figure 10. Go command: host side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 11. Go command: device side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Figure 12. Write Memory command: host side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Figure 13. Write Memory command: device side. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Figure 14. Erase Memory command: host side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Figure 15. Erase Memory command: device side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Figure 16. Extended Erase Memory command: host side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Figure 17. Extended Erase Memory command: device side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Figure 18. Write Protect command: host side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Figure 19. Write Protect command: device side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Figure 20. Write Unprotect command: host side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Figure 21. Write Unprotect command: device side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Figure 22. Readout Protect command: host side. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Figure 23. Readout Protect command: device side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Figure 24. Readout Unprotect command: host side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Figure 25. Readout Unprotect command: device side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

AN3155 USART bootloader code sequence

Doc ID 17066 Rev 2 5/37

1 USART bootloader code sequence

Figure 1. Bootloader for STM32 with USART

Once the system memory boot mode is entered and the STM32 microcontroller has been

configured (for more details refer to application note AN2606 “STM32 system memory boot

mode”) the bootloader code begins to scan the USARTx_RX line pin, waiting to receive the

0x7F data frame: one start bit, 0x7F data bits, even parity bit and one stop bit.

The duration of this data frame is measured using the Systick timer. The count value of the

timer is then used to calculate the corresponding baud rate factor with respect to the current

system clock.

Next, the code initializes the serial interface accordingly. Using this calculated baud rate, an

acknowledge byte (0x79) is returned to the host, which signals that the STM32 is ready to

receive commands.

X&RECEIVEDON

53!24X2XPIN

7AITFORA

COMMAND

'%4CMD

ROUTINE

AI

53!24XSELECTED

!UTOBAUDRATESEQUENCE

SEND!#+BYTEDISABLE

UNUSEDPERIPHERALS

2$CMD

ROUTINE

OPTIONAL

2OUTINESFOR

LOADING

INTO2!-

'/CMD

ROUTINE

*0TO?!DDRESS

'/CMD

#OMMAND

RECEIVED

'%4CMD

Choosing the USARTx baud rate AN3155

6/37 Doc ID 17066 Rev 2

2 Choosing the USARTx baud rate

The calculation of the serial baud rate for USARTx, from the length of the first byte that is

received, is used to operate the bootloader within a wide range of baud rates. However, the

upper and lower limits have to be kept, in order to ensure proper data transfer.

For a correct data transfer from the host to the microcontroller, the maximum deviation

between the internal initialized baud rate for USARTx and the real baud rate of the host

should be below 2.5%. The deviation (fB, in percent) between the host baud rate and the

microcontroller baud rate can be calculated using the formula below:

, where fB≤2.5%.

This baud rate deviation is a nonlinear function depending on the CPU clock and the baud

rate of the host. The maximum of the function (fB) increases with the host baud rate. This is

due to the smaller baud rate prescale factors, and the implied higher quantization error.

2.1 Minimum baud rate

The lowest tested baud rate (BLow) is 1200. Baud rates below BLow would cause the SysTick

timer to overflow. In this event, USARTx would not be correctly initialized.

2.2 Maximum baud rate

BHigh is the highest baud rate for which the deviation still does not exceed the limit. All baud

rates between BLow and BHigh are below the deviation limit.

The highest tested baud rate (BHigh) is 115 200.

fBSTM32 baud rate Host baud rate–

STM32 baud rate

------------------------------------------------------------------------------------------- 100%×=

AN3155 Bootloader command set

Doc ID 17066 Rev 2 7/37

3 Bootloader command set

The supported commands are listed in Ta bl e 1 below. Each command is further described in

this section.

Communication safety

All communications from the programming tool (PC) to the device are verified by:

1. 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

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

3. UART: parity check active (even parity)

Table 1. USART bootloader commands

Command(1)

1. If a denied command is received or an error occurs during the command execution, the bootloader sends

NACK byte and goes back to command checking.

Command code Command description

Get(2)

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.

0x00 Gets the version and the allowed commands supported

by the current version of the bootloader

Get Version & Read

Protection Status(2) 0x01 Gets the bootloader version and the Read Protection

status of the Flash memory

Get ID(2) 0x02 Gets the chip ID

Read Memory 0x11 Reads up to 256 bytes of memory starting from an

address specified by the application

Go 0x21 Jumps to user application code located in the internal

Flash memory or in SRAM

Write Memory 0x31 Writes up to 256 bytes to the RAM or Flash memory

starting from an address specified by the application

Erase(3)

3. Erase (x043) and Extended Erase (0x44) are exclusive. A device may support either the Erase command

or the Extended Erase command but not both.

0x43 Erases from one to all the Flash memory pages

Extended Erase(3) 0x44

Erases from one to all the Flash memory pages using

two byte addressing mode (available only for v3.0 usart

bootloader versions and above).

Write Protect(4)

4. See Section 3.1: Device-dependent bootloader parameters.

0x63 Enables the write protection for some sectors

Write Unprotect(4) 0x73 Disables the write protection for all Flash memory sectors

Readout Protect 0x82 Enables the read protection

Readout Unprotect(2) 0x92 Disables the read protection

Bootloader command set AN3155

8/37 Doc ID 17066 Rev 2

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

●ACK = 0x79

●NACK = 0x1F

3.1 Device-dependent bootloader parameters

While the USART bootloader protocol’s command set and sequences are the same for all

the STM32 devices, some parameters are device-dependent. For a few commands, the

value of some parameters may depend on the device used. The concerned parameters are

listed below:

●PID (product ID), which changes with the device

●Valid memory addresses (RAM, Flash memory, system memory, option byte areas)

accepted by the bootloader when the Read Memory, Go and Write Memory commands

are executed.

●Size of the Flash memory sector used when executing the Write Protect command.

For more details about the value of these parameters for the device you are using, please

refer to the “Device-dependent bootloader parameters” section in the “STM32 system

memory boot mode” application note (AN2606).

AN3155 Bootloader command set

Doc ID 17066 Rev 2 9/37

3.2 Get command

The Get command allows 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 host, as described in Figure 2.

Figure 2. Get command: host side

Figure 3. Get command: device side

Send 0x00 + 0xFF

Start Get

Wait for ACK

or NACK

Receive the number of bytes

(version+commands)

Receive the bootloader version

Receive the supported commands

Wait for ACK

or NACK

End of Get

NACK

ACK

ai14631

NACK

ACK

Send ACK byte

Start Get

Received

byte = 0x00+0xFF?

Send the number of bytes

(version+commands)

Send the bootloader version

Send the supported commands

End of Get

Ye s

ai14632

Send NACK byte

Send ACK byte

Bootloader command set AN3155

10/37 Doc ID 17066 Rev 2

The STM32 sends the bytes as follows:

3.3 Get Version & Read Protection Status command

The Get Version & Read Protection Status command is used to get the bootloader version

and the read protection status. When the bootloader receives the command, it transmits the

information described below (version, read protection: number of times it was enabled and

disabled) to the host.

Byte 1: ACK

Byte 2: N = 11 = the number of bytes to follow – 1 except current and ACKs.

Byte 3: Bootloader version (0 < Version < 255), example: 0x10 = Version 1.0

Byte 4: 0x00 – Get command

Byte 5: 0x01 – Get Version and Read Protection Status

Byte 6: 0x02 –GetID

Byte 7: 0x11 – Read Memory command

Byte 8: 0x21 – Go command

Byte 9: 0x31 – Write Memory command

Byte 10: 0x43 or

0x44

– Erase command or Extended Erase command (these commands are

exlusive)

Byte 11: 0x63 – Write Protect command

Byte 12: 0x73 – Write Unprotect command

Byte 13: 0x82 – Readout Protect command

Byte 14: 0x92 – Readout Unprotect command

Last byte (15): ACK

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