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 10nF ceramic capacitor 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. Stray capacitance (Cs), which includes PCB trace, pin, and parasitic capacitances, was estimated at 5 pF, a typical value for a 2-layer FR-4 board with short crystal traces.

To find the required capacitor value C for C12 and C13, the formula C = 2 × (CL − Cs) was applied. Substituting the values gives: 2 × (30 pF − 5 pF) = 50 pF. The nearest standard capacitor value, 51 pF, was selected for C12 and C13.

UART Pullup Resistors

The dedicated UART pins PA2, PA3, PB10, PB11, PA9, PA10 are tied to the 3.3V rail via the resistors (R7–R12). This prevents these pins from floating into an unknown state during device power-up/reset which may be interpreted as a transmission of garbage data. A 10K resistor value was chosen as it provides stable signal integrity while drawing a small amount of current (~0.33 mA) when the pin is driven low.

Debug and LED Circuit

The LEDs (D2, D3, and D4) are utilised for visual debugging and status checks. Each LED anode is tied to the 3.3V rail via its respective current-limiting resistor (R4, R5, and R6). The cathodes are driven via the microcontroller pins PB8, PA1 and PC0. The LEDs are illuminated by driving the microcontroller pins low (0V), sinking the current.

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 R6 were mapped accordingly.