Memory budget and peripheral selection
The STM32F038F6P6TR: 32 KB Flash and 4 KB SRAM are the hard ceiling for firmware and runtime data. For a simple PID loop with a Modbus RTU stack over UART, expect about 12-16 KB Flash and 1.5-2 KB SRAM consumed, leaving headroom for a bootloader and a small lookup table. The 9-channel 12-bit ADC samples analog inputs at up to 1 µs conversion time — adequate for reading a thermistor or a potentiometer at control-loop rates.
Board-level integration and supply rails
Decouple Vdd with a 100 nF ceramic placed within 3 mm of each supply pin; the 1.65 V minimum supply means a single 1.8 V rail serves both core and I/O, so no separate VddA/Vdda split is needed unless the ADC is used at full 12-bit resolution. For designs that see sustained 85°C case temperature, derate the Flash write endurance (typically 10 kcycles at 85°C vs 100 kcycles at 25°C per ST datasheet).