DS80C320 Datasheet, PDF (11/42 Pages) Dallas Semiconductor – High-Speed/Low-Power Micro

DS80C320/DS80C323
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
MD2 MD1 MD0
0
0
0
0
0
1
0
1
0
0
1
1
1
0
0
1
0
1
1
1
0
1
1
1
MEMORY
CYCLES
2
3 (default)
4
5
6
7
8
9
RD or WR STROBE
WIDTH IN CLOCKS
2
4
8
12
16
20
24
28
STROBE WIDTH
TIME @ 25 MHz
80 ns
160 ns
320 ns
480 ns
640 ns
800 ns
960 ns
1120 ns
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