20 MHz R8C core, 8 KB flash — the firmware budget
The R5F21182NP#U0 is a 16-bit R8C microcontroller from the R8C/1x/18 series, clocked at 20 MHz with 8 KB of program flash and 512 bytes of RAM. The 8 KB flash is the total user space — the bootloader does not carve out a hidden sector, so what you see is what you get for the application image. The 512-byte RAM split means the stack and interrupt vectors compete with the data buffer: a deep call tree or a large interrupt service routine will push into the 256-byte boundary quickly.
Peripheral set and I/O count — what connects where
Thirteen general-purpose I/O pins share the package with a SIO and a UART/USART serial interface. The on-chip peripherals include a voltage detect circuit, a power-on reset, a watchdog timer, and an LED drive capability — the POR and voltage detect eliminate the external supervisor IC for most low-cost designs. The WDT can be configured to generate a reset or an interrupt, which matters for safety-critical loops where a timeout must not immediately reset the system. The pad is the primary ground return for the core; a missing via under the pad lifts the die temperature above the rated junction limit in continuous operation.
Active production, ROHS3 — no last-time-buy pressure
The 8 KB flash and 512-byte RAM limit the application scope to simple control loops, sensor polling, or communication bridges — not a complex RTOS. For a BOM that needs a 16-bit core with serial I/O and internal oscillator, this part fits without a board spin for an external clock source.