I am designing a hobby electronic for first time, using STM32 to control a soldering tip. I read many documents of PCB layout, and also many information from this forum. And this is my first result, I am going to let this design be manufactured by PCB manufacture.
Since this is my first attempt, I would like to have some advice to check whether I'm doing wrong or not, before I send this design out to the factory.
This PCB will be a double layer PCB.
The components are going to be soldered by hand.
I am designing the PCB with EAGLE education version. (2 layers only)
From this document I learned :
- To have a GND island in bottom layer and guard ring on top layer to protect the OSC signal.
- The isolated ground island should be connected to the nearest MCU ground.
- The guard ring should be stitched by via to the ground island.
- No signal should run through the isolated ground area.
- The signal paths of OSC should be symmetrical as possible.
- The signal paths of OSC should be short as possible.
- Return paths of load C goes through via to Ground island
My OSC is running in 8 Mhz; load C are 18 pF.
I hope I understood the rule correctly, and also do the layout correct in the range of a hobby board.
Power and Decoupling C:
I am using 0603 cap. I want to keep the ground plane as whole as possible, so I don't want the signal trace go to the bottom layer. But I also can not keep the decoupling C on the top layer. That's why I moved decoupling C to the bottom layer. If any idea can be provided, which can make both traces and decoupling C on the top layer, will be very appreciated.
What I got as rules:
- The decoupling C should be placed as close as possible to the VDD/VSS pair.
- Power first go through the decoupling C then to the VDD/VSS pins
- MCU has local +3V3 and GND. And they are feed from a single point.
- Keep the ground plan not be cut.
- For the VDDA, a ferrite bead is needed.
- If multiple C are needed, place the C with smaller value closer the the VDD/VSS pair.
Please let me know, whether my layout is reasonable.
for my application a thermocouple signal is needed, which is in the soldering iron tip. The the tip has a heater resistor and a thermocouple inside and the thermocouple and heat resistor are sharing a common return path. The thermocouple voltage is measured in the period, when the heater voltage is not applied.
I am using a very simple non-inverted op amp to amplify the signal. What I am concerning about are:
- whether the return current of heater element will give big disturbance to the MCU. (Since the thermocouple voltage is only measured when no heater current flows, it doesn't matter that the current is effecting the op amp)
- Is it better to tie the OP amp VSS to the ground plane directly, or tie it to the thermocouple(-) as I did in the design? Or other options?
I am using a STM32F103C8T6. According to datasheet, .1uF and 2x 10uF for VDD/VSS pair. For fast signal I placed resistor for raising edge suppression. A cap is placed for filtering reset line. I am using SWIO for debugging port with SDO tracing.
Following sections is my current PCB design:
-Soldering Tip construction:
I hope the information I provided is enough to generate some feedback.
And also let me know, whether my understanding of design rules are correct.
Thanks a lot in advance.