I have device with an STM32F401 and a SIM800L. When the SIM800L sends data I have noise (usually 1 byte) received by the STM32 on UART2 connected to the SIM800. If I use FTDI instead of STM32 I never have noise.


The signal on UART2 RX looks clean. During the test the device was powered 4.0 V from a laboratory power supply (instead of a battery). The power on 5 V, 3V3 and VB looks good. A tantalum capacitor of 470 μF between 5 V and GND, or 3V3 and GND changes nothing. Replacing the SIM800L onboard capacitor from 100 μF to 470 μF decreases noise (instead of every time it appears seldom).


Overview: RX yellow, TX green

UART2 RX yellow, TX green

RX Noise

UART2 noise

RX Signal


UART2 RX never going lover than 2.2V during "noise". Input above 1.88V should be interpreted as HI.

Why do I have noise on UART2?

I have seen this post, and found it useful, nevertheless I rather want to understand why I have noise than blindly add a lot of capacitors.


Assuming that my STM32 instance incorrectly detects the input signal level, I tested it by sending a signal from FTDI to UART2, smoothly changing its lower limit. STM32 began to receive a signal at a lower limit of 1.2V.

Even more confused. Any ideas please?

Nothing received on UART2

Nothing received on UART2

Correct input on UART2

Correct input on UART2

  • 1
    \$\begingroup\$ Which bit is the noisy one? is it on the screenshot you uploaded? I want to see what you mean by 'noise'. What is the peak to peak voltage range? I would like to see an oscilloscope screenhot of that. The screenshot you uploaded seems clean. \$\endgroup\$ Oct 10, 2022 at 7:12
  • 5
    \$\begingroup\$ On screenshot normal response from sim800l. Noice appears before. I can see small fluctuations on RX, but oscilloscope didn't catch them if trigger set on 2V. (V_IH = 0.49 * Vcc + 0.26 = 1.88 V). I'll try catch the noice and add screenshot in the evening \$\endgroup\$
    – Sandre
    Oct 10, 2022 at 7:23

1 Answer 1


Try this if you can. Add 150pf(or pf values) capacitor to STM32's uart pins. Add nf, pf and nearly 1uf capacitor to supply. In high frequency RF applications use many nf-pf capacitors and put smallest one in near of supply pin. and increase values etc VDD->100pf 100nf 100nf 1uf 10uf.

  • 1
    \$\begingroup\$ good way of killing communication at higher frequencies. Adding capacitors on UART pins makes no sense at all \$\endgroup\$ Oct 11, 2022 at 8:17

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