PIC18F6X2X datasheet(79/386 Pages) MICROCHIP

79 / 386 page

 2003 Microchip Technology Inc.

Advance Information

DS39612A-page 77

PIC18F6X2X/8X2X

6.2.3

16-BIT BYTE SELECT MODE

Figure 6-3 shows an example of 16-bit Byte Select

mode for PIC18F8X2X devices. This mode allows table

write operations to word-wide external memories with

byte selection capability. This generally includes both

word-wide FLASH and SRAM devices.

During a TBLWT cycle, the TABLAT data is presented

on the upper and lower byte of the AD15:AD0 bus. The

WRH signal is strobed for each write cycle; the WRL

pin is not used. The BA0 or UB/LB signals are used to

select the byte to be written based on the Least

Significant bit of the TBLPTR register.

FLASH and SRAM devices use different control signal

combinations to implement Byte Select mode. JEDEC

standard FLASH memories require that a controller I/O

port pin be connected to the memory’s BYTE/WORD

pin to provide the select signal. They also use the BA0

signal from the controller as a byte address. JEDEC

standard static RAM memories, on the other hand, use

the UB or LB signals to select the byte.

FIGURE 6-3:

16-BIT BYTE SELECT MODE EXAMPLE

AD<7:0>

PIC18F8X2X

AD<15:8>

ALE

373

A<20:1>

373

WRH

A<19:16>

WRL

BA0

JEDEC Word

A<x:1>

D<15:0>

A<20:1>

D<15:0>

I/O

OE WR(1)

BYTE/WORD

FLASH Memory

JEDEC Word

A<x:1>

D<15:0>

WR(1)

SRAM Memory

138(2)

Address Bus

Data Bus

Control Lines

Note

This signal only applies to table writes. See Section 5.1, “Table Reads and Writes”.

2: Demultiplexing is only required when multiple memory devices are accessed.

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