16 KB flash, 8 KB RAM — the firmware budget
The STM32L031F4P6 carries 16 KB of program flash and 8 KB of SRAM (8K x 8). That flash footprint is tight: a minimal HAL plus a UART bootloader chews about 4-6 KB, leaving 10-12 KB for the application — enough for a sensor hub polling I²C and driving a PWM output, but not for a full RTOS with a TCP/IP stack. The 1 KB on-chip EEPROM (1K x 8) stores calibration constants and fault logs without an external serial EEPROM — useful when the BOM count is tight and the board has no spare I²C bus.
32 MHz Cortex-M0+ — the clock tree decision
The core runs at 32 MHz, the maximum for this STM32L0 part. The internal oscillator saves an external crystal if the application tolerates ±2% accuracy across temperature — if you need USB or a precise baud rate, budget for the HSE crystal and the two load caps. At 1.8 V the flash access inserts a wait state — the effective throughput drops below 32 MHz, so benchmark your tight loops at the actual VDD.
15 I/O in a 20-TSSOP — layout constraints
The 0.65 mm pitch routes cleanly on a two-layer board with 6/6 mil trace/space.