ADDRESS SPACES

Table 3-2. Minimum Times to Fetch Two Bytes of Code

Type of Code Memory

State Times

 

 

On-chip Code Memory

1

 

 

External Memory (page mode)

2

 

 

External Memory (nonpage mode)

4

 

 

NOTE

If your program executes exclusively from on-chip ROM/OTPROM/EPROM (not from external memory), beware of executing code from the upper eight bytes of the on-chip ROM/OTPROM/EPROM (FF:1FF8H–FF:1FFFH for 8 Kbytes, FF:3FF8H–FF:3FFFH for 16 Kbytes). Because of its pipeline capability, the 8XC251Sx may attempt to prefetch code from external memory (at an address above FF:1FFFH/FF:3FFFH) and thereby disrupt I/O ports 0 and 2. Fetching code constants from these eight bytes does not affect ports 0 and 2.

If your program executes from both on-chip ROM/OTPROM/EPROM and external memory, your code can be placed in the upper eight bytes of the on- chip ROM/OTPROM/EPROM. As the 8XC251Sx fetches bytes above the top address in the on-chip ROM/OTPROM/EPROM, the code fetches automati- cally become external bus cycles. In other words, the rollover from on-chip ROM/OTPROM/EPROM to external code memory is transparent to the user.

3.2.2.1Accessing On-chip Code Memory in Region 00:

The 87C251SB, SQ and the 83C251SB, SQ can be configured so that the upper half of the 16- Kbyte on-chip code memory can also be read as data at locations in the top of region 00: (see “Configuration Bytes” on page 14-7). That is, locations FF:2000H–FF:3FFFH can also be access- ed at locations 00:E000H–00:FFFFH. This is useful for accessing code constants stored in ROM/OTPROM/EPROM. Note, however, that all of the following three conditions must hold for this mapping to be effective:

•The device is configured with EMAP# = 0 in the UCONFIG1 register (See Chapter 4).

•EA# = 1.

•The access to this area of region 00: is a data read, not a code fetch.

If one or more of these conditions do not hold, accesses to the locations in region 00: are referred to external memory.

NOTE

Remapping does not apply to the 87C251SA, SP and the 83C251SA, SP.

3-9

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Intel 8XC251SP Minimum Times to Fetch Two Bytes of Code, Accessing On-chip Code Memory in Region, Type of Code Memory
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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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