MINIMUM HARDWARE SETUP

While the RST pin is high ALE, PSEN#, and the port pins are weakly pulled high. The first ALE occurs 32TOSC after the reset signal goes low. For this reason, other devices can not be synchro- nized to the internal timings of the 8XC251Sx.

NOTE

Externally driving the ALE and/or PSEN# pins to 0 during the reset routine may cause the device to go into an indeterminate state.

Powering up the 8XC251Sx without a reset may improperly initialize the program counter and SFRs and cause the CPU to execute instructions from an undetermined memory location.

11.4.4 Power-on Reset

To automatically generate a reset at power-on, connect the RST pin to the VCC pin through a 1-µF capacitor as shown in Figure 11-1.

When VCC is applied, the RST pin rises to VCC, then decays exponentially as the capacitor charg- es. The time constant must be such that RST remains high (above the turn-off threshold of the Schmitt trigger) long enough for the oscillator to start and stabilize, plus 64TOSC. At power-on, VCC should rise within approximately 10 ms. Oscillator start-up time is a function the crystal fre- quency; typical start-up times are 1 ms for a 10 MHz crystal and 10 ms for a 1 Mhz crystal.

During power-on, the port pins are in a random state until forced to their reset state by the asyn- chronous logic.

Reducing VCC quickly to 0 causes the RST pin voltage to momentarily fall below 0 V. This volt- age is internally limited and does not harm the device.

11-7

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Intel 8XC251SA, 8XC251SP, 8XC251SQ, 8XC251SB, Embedded Microcontroller manual Power-on Reset

Embedded Microcontroller, 8XC251SP, 8XC251SA, 8XC251SQ, 8XC251SB specifications

The Intel 8XC251 series of embedded microcontrollers is a family of versatile and powerful devices, designed to meet the demands of a wide range of applications. With models such as the 8XC251SB, 8XC251SQ, 8XC251SA, and 8XC251SP, this series offers unique features while maintaining a high level of performance and reliability.

At the heart of the 8XC251 microcontrollers is the 8051 architecture, which provides a 16-bit processor capable of executing complex instructions efficiently. This architecture not only allows for a rich instruction set but also facilitates programming in assembly language and higher-level languages like C, which are essential for developing sophisticated embedded systems.

One of the significant features of the 8XC251 family is its integrated peripherals, including timer/counters, serial communication interfaces, and interrupt systems. These peripherals enable developers to implement timing functions, data communication, and real-time processing, all of which are crucial in modern embedded applications. The 8XC251SB and 8XC251SQ models, for instance, come equipped with multiple I/O ports that allow for interfacing with other devices and systems, enhancing their functionality in various environments.

The memory architecture of the 8XC251 devices is noteworthy, featuring on-chip ROM, RAM, and EEPROM. The on-chip memory allows for fast access times, which is essential for executing programs efficiently. Moreover, the EEPROM serves as non-volatile memory, enabling the storage of configuration settings and important data that must be retained even when power is lost.

In terms of operating voltage, the 8XC251 devices are designed to operate in a wide range, typically between 4.0V and 6.0V. This flexibility makes them suitable for battery-powered applications, where energy efficiency is critical. The power management features, including reduced power modes, further enhance their suitability for portable devices.

Lastly, the 8XC251 series is supported by a wide range of development tools and resources, allowing engineers and developers to streamline the development process. This support, combined with the microcontrollers' robust features, makes the Intel 8XC251 family a reliable choice for various embedded applications, such as industrial automation, automotive systems, and consumer electronics.

Overall, the Intel 8XC251SB, 8XC251SQ, 8XC251SA, and 8XC251SP deliver high performance, versatility, and ease of use, making them a preferred choice for embedded system designers looking to develop efficient and effective solutions.