DS80C320 datasheet(11/42 Pages) DALLAS

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DS80C320/DS80C323

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On reset, the Stretch value will default to a 1, resulting in a three-cycle MOVX. Therefore, RAM access

will not be performed at full speed. This is a convenience to existing designs that may not have fast RAM

in place. When maximum speed is desired, the software should select a Stretch value of 0. When using

very slow RAM or peripherals, a larger stretch value can be selected. Note that this affects data memory

only and the only way to slow program memory (ROM) access is to use a slower crystal.

Using a Stretch value between 1 and 7 causes the microcontroller to stretch the read/write strobe and all

related timing. This results in a wider read/write strobe allowing more time for memory/peripherals to

respond. The timing of the variable speed MOVX is shown in the Electrical Specifications. Note that full

speed access is not the reset default case. Table 3 below shows the resulting strobe widths for each

Stretch value. The memory stretch is implemented using the Clock Control Special Function Register at

SFR location 8Eh. The stretch value is selected using bits CKCON.2-0. In the table, these bits are referred

to as M2 through M0. The first stretch (default) allows the use of common 120 ns or 150 ns RAMs

without dramatically lengthening the memory access.

DATA MEMORY CYCLE STRETCH VALUES Table 3

CKCON.2-0

MEMORY

or WR STROBE

STROBE WIDTH

MD2

MD1

MD0

CYCLES

WIDTH IN CLOCKS

TIME @ 25 MHz

80 ns

3 (default)

160 ns

320 ns

480 ns

640 ns

800 ns

960 ns

1120 ns

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