8XC251SA, SB, SP, SQ USER’S MANUAL

7.6QUASI-BIDIRECTIONAL PORT OPERATION

Port 1, port 2, and port 3 have fixed internal pullups and are referred to as “quasi-bidirectional” ports. When configured as an input, the pin impedance appears as logic one and sources current (see the 8XC251Sx datasheet) in response to an external logic-zero condition. Port 0 is a “true bidirectional” pin. The pin floats when configured as input. Resets write logical one to all port latches. If logical zero is subsequently written to a port latch, it can be returned to input conditions by a logical one written to the latch. For additional electrical information, refer to the 8XC251SA, SB, SP, SQ High-Performance CHMOS Microcontroller Datasheet.

NOTE

Port latch values change near the end of read-modify-write instruction cycles. Output buffers (and therefore the pin state) update early in the instruction after the read-modify-write instruction cycle.

Logical zero-to-one transitions in port 1, port 2, and port 3 utilize an additional pullup to aid this logic transition (see Figure 7-4). This increases switch speed. The extra pullup briefly sources 100 times the normal internal circuit current. The internal pullups are field-effect transistors rather than linear resistors. Pullups consist of three p-channel FET (pFET) devices. A pFET is on when the gate senses logical zero and off when the gate senses logical one. pFET #1 is turned on for two oscillator periods immediately after a zero-to-one transition in the port latch. A logic one at the port pin turns on pFET #3 (a weak pullup) through the inverter. This inverter and pFET pair form a latch to drive logic one. pFET #2 is a very weak pullup switched on whenever the associ- ated nFET is switched off. This is the traditional CMOS switch convention. Current strengths are 1/10 that of pFET #3.

7-6

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Intel 8XC251SB, 8XC251SA, 8XC251SP, 8XC251SQ, Embedded Microcontroller manual QUASI-BIDIRECTIONAL Port Operation
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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.
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