STM32 automatic ISP circuit design

STM32 supports emulator and serial port downloader. The content to be introduced belongs to serial port download, which is what we usually call ISP download.

We already know that manual ISP download program can be realized by controlling BOOT0 pin. When STM32 is powered on, it will automatically detect the level of BOOT0 pin. If it is high level, it will wait for the user to download the program; if it is low level, it will run the program previously downloaded to the microcontroller by the user. So we need to lead out the BOOT0 pin, and then control it to ground or connect to VCC to download or run the program. During the debugging process, we need to constantly control BOOT0, which is very troublesome. Then, automatic ISP should come into play.

Automatic ISP, connect BOOT0 directly to the ground, then the program will run every time the power is turned on, and as long as you click the "Download" button on the computer, the program will start downloading, and after the program is downloaded, it will start executing. To achieve this goal, you need to use the serial port handshake signals DTR and RTS.

The following is the free ISP download software developed by Shenzhen Pengyuan Diantong Technology Co., Ltd. Please see the part in the red box.

DTR is connected to RESET (reset pin) to control reset, and RTS is connected to BOOT0 to control program running or waiting for downloading.

The following describes how to use the USB to TTL chip to implement STM32 automatic ISP. The CH340 chip is shown in the figure below:

In CH340 chip, DTR and RTS pins are usually high level and low level is effective. Because the RESET pin of STM32 is also usually high level and low level is reset, DTR can be directly connected to RESET. However, BOOT0 is high level to download the program and low level to run the program, which is exactly the opposite of RTS, so we need to invert it and add an NPN transistor. The circuit is shown in the figure below:

ISP software selection:

After seeing the options in the above picture, some people will have doubts. In theory, it should be a low-level reset, and then a low-level entry into the BOOTLoader. However, one thing needs to be noted, the high and low levels mentioned here are for the original 9-pin serial port of the computer, that is, "232 level", and the USB to TTL chip we use is "TTL level", which is exactly the opposite. For this part, you can refer to another article I wrote: http://www.rationmcu.com/elecjc/947.html

The download process and results are shown in the following figure:

Note: When CH340 is just powered on, it takes a few seconds to stabilize. During this period, the DTR pin will become low twice or three times, which will cause the microcontroller to reset two or three times after powering on. After stabilization, it will not affect the program operation. If you do not want the microcontroller to reset several times after powering on, just disconnect DTR and RESET when powering on.