STM32 IAP upgrade based on Xmodem [Transfer]

Experimental platform: Windows 7 + STM32F103ZET6
Experimental purpose: You don't need any burning tools to remotely upgrade the code of your product!
1. Introduction to Xmodem protocol
2. IAP programming principle
3. Boot and App programming
4. Experimental steps
5. Remote upgrade application

1. Modem protocol

The main file transfer protocols for serial communication are: Xmodem, Ymodem, Zmodem and KERMIT.

Xmodem protocol transmission is completed by the receiving program and the sending program. The receiving program first sends a negotiation character to negotiate the verification method. After the negotiation is passed, the sending program starts to send data packets. After the receiving program receives a complete data packet, it verifies the data packet according to the negotiated method. After the verification is passed, a confirmation character is sent, and then the sending program continues to send the next packet; if the verification fails, a denial character is sent, and the sending program retransmits the data packet.
The Xmodem protocol was proposed and implemented by Ward Chritensen in the 1970s. The unit of transmitted data is information packets.
The Xmodem protocol generally supports 128-byte data packets, and supports two verification methods: checksum and CRC.

2. IAP programming principles

There are currently two ways to implement online programming: In-System Programming (ISP) and In-Application Programming (IAP). ISP generally programs the entire internal Flash memory through the microcontroller's dedicated serial port, while IAP technology structurally divides the Flash memory into two parts. When running a user program on one memory bank, the other memory bank can be reprogrammed, and then the control is transferred from one memory bank to another. The implementation of IAP is more flexible: usually the serial port, network port, USB and other interfaces of the microcontroller can be used for online programming, thereby realizing remote code upgrades. 

Some users hope that the product can automatically update the program content through some remote communication method during actual application. Obviously, ISP can no longer meet such needs. The newly emerged IAP burning method provides an effective means for self-update of the program. The Flash memory inside the microcontroller stores the user's program code, which cannot be modified during normal operation. However, with IAP, the user program can modify part or even all of the program code as needed (if certain conditions are met). The new program code may be automatically generated during the program operation, or it may come from a remote device.

The Boot code must be burned in through other means, such as JTAG or ISP; the APP code can be burned in using the IAP function of the Boot code, and the IAP function can be used to easily update the code in the future.　

These two parts of code are stored in different address ranges of FLASH. Generally, the Bootloader is stored in the lowest address area, followed by the APP program (note that if the FLASH capacity is sufficient, many APP programs can be designed).

Interrupt vector table offset setting: (in misc.c)
void NVIC_SetVectorTable(uint32_t NVIC_VectTab, uint32_t Offset)
#ifdef VECT_TAB_SRAM
SCB->VTOR = SRAM_BASE | VECT_TAB_OFFSET;
#else
SCB->VTOR = FLASH_BASE | VECT_TAB_OFFSET;
#endif
Design VECT_TAB_OFFSET=0x4000 Reserve 16KB of Flash space for the Boot program.

3. Experimental steps:
Experimental verification through the STM3210e development board:
The Boot program implements the following functions:

1. Implement in-application programming (IAP) through the serial port.
 

2. Download the hex file (CRC or SUM) to the specified Flash storage area.
 
3. Erase any specified sector (8~127).