Made an effort to learn board design skills from scratch
Goal : To design a STM32 breakout board with bluetooth support.
[Coming soon.. are the Schematic and Board files. ]
- All pins should be accessible
- Board should be able to power up using external 12V battery
- Chip should be able to get programmed with JTAG
Skills Used/ Learnt :
Eagle Software, PCB Layout definitions, 0805 SMD Soldering
Finalizing the schematic features : The board should contain the following features
Power : A step down 12V -> 3.3V power converter and regulator.
The figure on the left is a schematic for obtaining 12V power from a battery (DC jack) and routing it on the board. The right side chip regulates 12V power to 3.3V
The TAG Connect connector takes in 3.3V and TDO, TMS and clock signals are interfaced to the microcontroller pins. This circuit is used to program the microcontroller
An external switch is provided to force reset the microcontroller
Crystals and Decoupling Caps
A decoupling capacitor is a capacitor used to decouple one part of an electrical network (circuit) from another. Noise caused by other circuit elements is shunted through the capacitor, reducing the effect it has on the rest of the circuit. While laying out the board, the decoupling caps should be placed as close as possible to the pins being decoupled.
The right side circuit is oscillator circuit. This is typically used to create an electrical signal with precise frequency needed for keeping time, provide stable clock signals etc. There are low freq 32.6KHz and high freq 925MHz) crystals used in this design.
Interfacing bluetooth chip is very easy. The appropriate TX and RX pins should be interfaced correctly with the UART TX and UART RX pins from the microcontroller. The chip requires power and ground and a clock signals. I used the RN41 bluetooth modem.
Key points I learnt while laying out a board are
- Never place all the components on the board at once. Things get messy
- Place each smaller circuit and route it (one circuit at a time).
- Place the components on a circuit like that in schematic. Plays out easy
- Use ratsnest very frequently. This simplifies the circuit
- Place decoupling caps as close to the pis as you can
Soldering SMD Components:
There are nice videos for the basics of surface mount soldering on Youtube. Definitely watch them for tips and tricks