18F8680 datasheet(95/496 Pages) MICROCHIP

95 / 496 page

 2004 Microchip Technology Inc.

DS30491C-page 93

PIC18F6585/8585/6680/8680

6.0

EXTERNAL MEMORY

INTERFACE

The external memory interface is a feature of the

PIC18F8X8X devices that allows the controller to

access external memory devices (such as Flash,

EPROM, SRAM, etc.) as program memory.

The physical implementation of the interface uses

27 pins. These pins are reserved for external address/

data bus functions; they are multiplexed with I/O port

pins on four ports. Three I/O ports are multiplexed with

the address/data bus, while the fourth port is multi-

plexed with the bus control signals. The I/O port func-

tions are enabled when the EBDIS bit in the MEMCON

multiplexed pins and their functions is provided in

Table 6-1.

As implemented in the PIC18F8X8X devices, the

interface operates in a similar manner to the external

memory interface introduced on PIC18C601/801

microcontrollers. The most notable difference is that

the interface on PIC18F8X8X devices only operates in

16-bit modes. The 8-bit mode is not supported.

For a more complete discussion of the operating modes

that use the external memory interface, refer to

Section 4.1.1

“PIC18F8X8X

Program

Memory

Modes”.

6.1

Program Memory Modes and the

External Memory Interface

As previously noted, PIC18F8X8X controllers are

capable of operating in any one of four program mem-

ory modes using combinations of on-chip and external

program memory. The functions of the multiplexed port

pins depend on the program memory mode selected as

well as the setting of the EBDIS bit.

In Microprocessor Mode, the external bus is always

active and the port pins have only the external bus

function.

In Microcontroller Mode, the bus is not active and the

pins have their port functions only. Writes to the

MEMCOM register are not permitted.

In Microprocessor with Boot Block or Extended

Microcontroller Mode, the external program memory

bus shares I/O port functions on the pins. When the

device is fetching or doing table read/table write

operations on the external program memory space, the

pins will have the external bus function. If the device is

fetching and accessing internal program memory

locations only, the EBDIS control bit will change the

pins from external memory to I/O port functions. When

EBDIS = 0, the pins function as the external bus. When

EBDIS = 1, the pins function as I/O ports.

Note:

The external memory interface is not

implemented on PIC18F6X8X (64/68-pin)

devices.

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