CHAPTER 13

EXTERNAL MEMORY INTERFACE

13.1 OVERVIEW

The external memory interface comprises the external bus (ports 0 and 2, and when enabled also includes port 1.7:6) as well as the bus control signals (RD#, WR#, PSEN# and ALE). Chip con- figuration bytes (see Chapter 4, “Device Configuration”) determine several interface options: page mode or nonpage mode for external code fetches; the number of external address bits (16, 17, or 18); the address ranges for RD#, WR#, and PSEN#; and the number of preprogrammed external wait states to extend RD#, WR#, PSEN# or ALE. Real-time wait states can be enabled with special function register WCON.1:0. You can use these options to tailor the interface to your application. See also section 4.5, “Configuring the External Memory Interface.”

The external memory interface operates in either page mode or nonpage mode. Page mode pro- vides increased performance by reducing the time for external code fetches. Page mode does not apply to code fetches from on-chip memory. The reset routine configures the 8XC251Sx for op- eration in page mode or nonpage mode according to bit 1 of configuration byte UCONFIG0. Fig- ure 13-1 shows the structure of the external address bus for page and nonpage mode operation. P0 carries address A7:0 while P2 carries address A15:8. Data D7:0 is multiplexed with A7:0 on P0 in nonpage mode and with A15:8 on P2 in page mode.

Table 13-1 describes the external memory interface signals. The address and data signals (AD7:0 on port 0 and A15:8 on port 2) are defined for nonpage mode.

8XC251SA

 

RAM/

8XC251SA

 

RAM/

 

EPROM/

 

EPROM/

8XC251SB

 

8XC251SB

 

 

Flash

 

Flash

8XC251SP

 

8XC251SP

 

 

 

 

 

8XC251SQ

A15:8

 

8XC251SQ

 

D7:0

 

 

 

 

 

 

 

 

 

P2

 

A15:8

 

 

 

 

AD7:0

A7:0

P2

Latch

A15:8

P0

Latch

A7:0

A15:8/D7:0

A15.8

 

 

 

D7:0

P0

 

A7:0

 

 

A7:0

 

 

 

 

 

 

 

 

Nonpage Mode

 

Page Mode

 

 

 

 

 

 

A4159-02

Figure 13-1. Bus Structure in Nonpage Mode and Page Mode

13-1

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Intel Embedded Microcontroller, 8XC251SA, 8XC251SP, 8XC251SQ, 8XC251SB manual Chapter External Memory Interface
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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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