ATMEL AT89C51RD2handbook

This document introduces the features of the AT89C51ED2 chip, including 80C52 compatibility, 8051 instruction compatibility, six 8-bit I/O ports, three 16-bit timers/counters, 256 bytes of erasable scratchpad RAM, 9 interrupt sources, 4 priority levels, integrated power monitor, ISP (in-system programming), 2048 bytes of boot ROM, etc.