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Joel R. , 04-11-2025, 08:02 PM

Hello, I'm designing a reset circuit for an STM32-based board and want to double-check the correct routing of components involved in the reset line.

Here’s my setup (Image 1 schematic):

- An SWD debug connector (with RESET pin)
- A 49.9Ω resistor between the MCU’s NRST pin and a TVS diode for ESD protection
- A push-button (manual reset) with a capacitor and pull-up resistor
- All these three subcircuits share a common net, which I call *RST_PIN* (in the images is *~{RST}_PIN*)

What I think the routing should be (Image 2 PCB layout):
1. MCU:
NRST pin (MCU) → 49.9Ω resistor → TVS cathode → *RST_PIN*
2. SWD:
*RST_PIN* also connects to SWD RESET pin
3. Push-Button Circuit:
Button + Capacitor + Pull-up → all tied to *RST_PIN*

My question: Should I route *RST_PIN* using a copper fill (Image 3) to tie all these nodes, or should I use traces that branch from a central node (Image 2)?
What if I want to maintain 50Ω controlled impedance for the RESET trace (my stackup requires a 6.16 mil trace width for 50Ω).
Can anyone confirm this layout is solid or suggest improvements? Thank you.

Joel R. , 04-11-2025, 08:06 PM

Image 3

QDrives , 04-11-2025, 08:36 PM

You want the TVS diode closest to the ESD causing sources (eg connector and button).
Small track from the TVS toward the MDC.
Capacitor closer to the MCU.