Power Supply Unit (PSU)

Linear PSU based on the ASM1117-3.3 voltage regulator was used to step down the +5V from the USB to 3.3V. C1 and C2 are the input and output smoothing capacitors. R1 is the 1.5kΩ current limiting resistor for the LED D1, which serves as the power indicator.

Microcontroller Decoupling Capacitors

In this design C3, C4, C5, C6 and C7 are decoupling capacitors for the microcontroller. Their primary function is to stabilize the supply voltage by acting as local energy reservoirs. During operation the microcontroller can draw sudden bursts of current due to internal switching. This current can briefly dip the voltage on the power rail potentially leading the microcontroller to brown out or cause erratic behavior. C4, C5, C6 and C7 are 100nf ceramic capacitors chosen for their effectiveness in filtering high frequency noise, C3 is a complementary 10 nano-farad ceramic capacitors included to further filter the higher frequencies and transients.

Analogue Voltage Stabilisation and Filtering

Ferrite beads and capacitors were used to stabilize the analog supply for precision components.

Crystal Oscillator Circuit

The crystal’s load capacitance (CL) was taken from the datasheet, which specifies a value of 30 pF. This is the capacitance required for the crystal to oscillate at its rated frequency. The stray capacitance (Cs), which includes PCB trace, pin, and parasitic capacitances, was estimated to be 5 pF.

Using the formula for two equal capacitors connected to ground, the required capacitor value is: CL = 2 × (CLo − Cs). Substituting the values gives:2 × (30 pF − 5 pF) = 50 pF.

The nearest standard capacitor value, 51 pF, was selected for C12 and C13.

Communication Interfaces

USB FS and UART headers were included for both firmware upload and serial debugging.

Debug and LED Circuit

The LEDs D2, D3, and D4 are included for quick debugging. Each LED is connected to the 3.3 V rail through its respective current-limiting resistor (R4, R5, and R6), and the cathodes are connected to microcontroller pins PB8, PA1, and PC0. Because the anodes are tied to 3.3 V, the LEDs are activated by driving the microcontroller pins low.

I/O Mapping

Common GPIOs were routed to headers for flexibility in testing peripherals. The LEDs D2, D3, and D4 along with their limiting resistors R4, R5, and R65 were mapped accordingly.