View 89c535 datasheet:

INTEGRATED CIRCUITS 89C535/89C536/89C538 CMOS single-chip 8-bit microcontrollers with FLASH program memory Preliminary specification 1997 June 05 IC20 Data Handbook Philips Semiconductors Philips Semiconductors Preliminary specification CMOS single-chip 8-bit microcontrollers 89C535/89C536/89C538 with FLASH program memory DESCRIPTION LOGIC SYMBOL The 89C535/89C536/89C538 are Single-Chip 8-Bit Microcontrollers VCC VSS manufactured in advanced CMOS process and are derivatives of the 80C51 microcontroller family. All the devices have the same XTAL1 instruction set as the 80C51. ADDRESS AND The devices also have four 8-bit I/O ports, three 16-bit timer/event DATA BUS counters, a multi-source, two-priority-level, nested interrupt structure, UART and on-chip oscillator an