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DS80C323-MCD Datasheet(PDF) 11 Page - Dallas Semiconductor |
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DS80C323-MCD Datasheet(HTML) 11 Page - Dallas Semiconductor |
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11 / 42 page  DS80C320/DS80C323 11 of 42 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 RD or WR STROBE STROBE WIDTH MD2 MD1 MD0 CYCLES WIDTH IN CLOCKS TIME @ 25 MHz 0 0 0 2 2 80 ns 0 0 1 3 (default) 4 160 ns 0 1 0 4 8 320 ns 0 1 1 5 12 480 ns 1 0 0 6 16 640 ns 1 0 1 7 20 800 ns 1 1 0 8 24 960 ns 1 1 1 9 28 1120 ns |
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