4 MHz core, OTP memory — the BOM-fit tier
The Microchip PIC16C621AT-04I/SO is an 8-bit PIC MCU from the PIC® 16C family, clocked at 4 MHz with 1.75 KB of one-time-programmable (OTP) program memory and 96 x 8 bytes of RAM. It packs 13 I/O lines, a brown-out detect/reset, power-on reset, and watchdog timer into an 18-pin SOIC package.
OTP: no field updates possible
The 1.75 KB program memory is OTP — once programmed, the code is permanent. This is the single most important sourcing decision on this part. The firmware must be final and fully validated before production programming; there is no erase-and-reprogram cycle. For designs that anticipate field updates or iterative firmware development, a Flash-based sibling like the PIC16LF1559-I/SS (32 MHz, Flash, 17 I/O) is a more flexible alternative, though it carries a different footprint and peripheral set.
At 4 MHz the PIC16C621AT-04I/SO delivers roughly 1 MIPS. That is adequate for slow sensor polling, relay timing, keypad scanning, or simple state machines — think a thermostat, a fan controller, or a garage-door opener. It will not run a PID loop at kilohertz rates or handle bit-banged high-speed serial. The 96-byte RAM limits buffering; keep your lookup tables and state variables lean.
Industrial temperature range and peripherals
The external oscillator requirement (no internal RC) means the BOM must include a crystal or ceramic resonator and its load caps.
That said, OTP parts see lower demand than their Flash equivalents, and the 4 MHz speed grade is a niche bin — independent distribution is the most reliable channel for this exact code.