The C501-L, C501-1R, and C501-1E described in this document are compatible (also pin compatible) with the 80C52 and can be used in typical 80C52 applications. The C501-1R contains a non-volatile 8K×8 read-only program memory, a volatile 256×8 read/write data memory, four ports, three 16-bit timers/counters, a seven source, two priority level interrupt structure and a serial port. The C501-L is identical, except that it lacks the program memory on chip. The C501-1E contains a one-time programmable (OTP) program memory on chip. The term C501 refers to all versions within this specification unless otherwise noted.