72 MHz Cortex-M3 in a 7×7 mm QFN — the board-fit story
The STM32F103CBU6 packs an ARM Cortex-M3 core running at 72 MHz into a 48-UFQFPN package with a 7×7 mm footprint and an exposed thermal pad. Without those vias, the 85 °C ambient ceiling derates the core frequency before the flash wait-state limit does. The 37 GPIOs are 5 V-tolerant on most pins, so a 5 V sensor or keypad can connect through a series resistor without a level shifter, but the ADC inputs (10-channel, 12-bit) must stay within the VDD range for full accuracy.
128 KB flash and 20 KB SRAM — what fits
With 128 KB of program flash and 20 KB of SRAM, the memory budget suits a motor-control algorithm plus a CANopen stack, or a USB virtual-com-port bridge with a modest data buffer. The 128 KB flash is organized as 64 pages of 2 KB each — the smallest erasable sector is 2 KB, so an OTA firmware update must manage page boundaries. The 20 KB SRAM leaves about 8-10 KB for the application heap after the RTOS (FreeRTOS or similar) and the peripheral DMA descriptors are allocated. The peripheral set includes a motor-control PWM timer with complementary outputs and programmable dead-time, plus a CAN 2.0B controller — this combination targets field-oriented control (FOC) of a BLDC or PMSM motor over a CAN bus. The USB 2.0 full-speed device interface shares the same 48 MHz clock derived from the PLL, so a USB-CDC or HID implementation does not require an external crystal if the internal oscillator is trimmed for the 0.25% accuracy USB demands.