The AT89S2051/S4051 is a low-voltage, high-performance CMOS 8-bit microcontroller with 2K/4K bytes of In-System Programmable (ISP) Flash program memory. The device is manufactured using Atmel's high-density nonvolatile memory technology and is compatible with the industry-standard MCS-51 instruction set. By combining a versatile 8-bit CPU with Flash on a monolithic chip, the Atmel AT89S2051/S4051 is a powerful microcontroller which provides a highly-flexible and cost-effective solution to many embedded control applications. Moreover, the AT89S2051/S4051 is designed to be function compatible with the AT89C2051/C4051 devices, respectively. The AT89S2051/S4051 provides the following standard features: 2K/4K bytes of Flash, 256 bytes of RAM, 15 I/O lines, two 16-bit timer/counters, a six-vector, fourlevel interrupt architecture, a full duplex enhanced serial port, a precision analog comparator, on-chip and clock circuitry. Hardware support for PWM with 8-bit resolution and 8-bit prescaler is available by reconfiguring the two on-chip timer/counters. In addition, the AT89S2051/S4051 is designed with static logic for operation down to zero frequency and supports two software-selectable power saving modes. The Idle Mode stops the CPU while allowing the RAM, timer/counters, serial port and interrupt system to continue functioning. The power-down mode saves the RAM contents but freezes the disabling all other chip functions until the next external interrupt or hardware reset.