reset circuit problem with stm32f4

[Moamen Ayman]

Posted on July 02, 2017 at 19:29

Hello, I'm using stm32f407vgt6 MCU in a control system and I'm facing a strange problem in the reset circuit, when I power on the system it doesn't start until I put a short circuit wire manually between reset pin and GND and then remove this wire so system can start properly. system's external reset circuit consists of pulldown 0.1uF capacitor and pullup 10KOhm resistor. I have tried to replace the 10KOhm resistor with 100KOhm but nothing changed. I also replaced the 0.1uF capacitor with 1uF and also nothing changed. Could someone help me to find out what may cause this problem and why I need to connect the reset pin to ground for just a second in order for the system to start?

Thanks so much in advance

Note: this post was migrated and contained many threaded conversations, some content may be missing.

[Tesla DeLorean]

Posted on July 02, 2017 at 21:22

What else in your system connects to NRESET, or is dependent on the power supply coming up?

On a scope or analyzer how wide is the reset pulse as built? Can you plot that against the supplies in the system?

What does the part do when it starts? Does it run at all? Does it bring up other clocks, or try to interact with external parts?

Are the capacitors and voltages at VCAP correct?

Consider an external POR circuit thresholding your highest supply, using a MCP120 or similar part, holding the NRST low for several hundred milliseconds.

[Moamen Ayman]

Posted on July 03, 2017 at 00:05

There is only a track connecting the NRESET pin with SWD circuit so I can use it when re-flashing the firmware and by the way when downloading a new code, it starts working properly but when I restart the system by turning power off and then turning it on again it doesn't work and I must connect the reset pin to GND for a second in order for the system to start. As I described the NRST is connected to pulldown capacitor which is connected to system common ground in addition to a pullup resistor which is also connected to system common VDD (3v common with MCU VDD pins).

I will try to plot the reset and power signals on an oscilloscope but I don't think it will be easy to get the transient response of the system. 

Voltages on VCAP are 1.26v so I think they are correct.

I didn't try a POR circuit before, could you describe it in more details?

[Tesla DeLorean]

Posted on July 03, 2017 at 01:03

The MCP120 has 3 pins, VCC, GND and RESET output, pull a datasheet.

[bbee]

Posted on July 03, 2017 at 08:04

Check pins BOOT0 and BOOT1. Maybe they are floating after a POR and the bootloader gets started?

[Moamen Ayman]

Posted on July 04, 2017 at 03:49

I connected BOOT0 directly to GND and according to datasheet when BOOT0 is connected to GNG BOOT1 is don't care so I let it floating, do I need to connect BOOT1 to GND also?

[Moamen Ayman]

Posted on July 04, 2017 at 03:49

I will try using MCP120 with threshold of 3v

[Moamen Ayman]

Posted on July 04, 2017 at 04:00

But what I am finding strange is that this problem happens in some samples of the system and some other work properly without facing any issues, I talking about the same system design. so why? 

and I built many different designs with the same reset circuit and it's the first time to face such a problem!

[bbee]

Posted on July 04, 2017 at 08:04

IMHO it should be OK this way, as long as you don't plan to start into the bootloader.

Have a look at

https://community.st.com/0D50X00009XkYeESAV

Seems to be the same problem.

I wonder about the pull up resistor on NRST because usually there is an internal permanent pull up allready connected.

[Moamen Ayman]

Posted on July 04, 2017 at 21:12

ST uses a pullup 100KOhm resistor also with STM32F4-Discovery board.