DP8500 datasheet(18/70 Pages) NSC

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40 Refresh Options (Continued)

43 EXTENDING REFRESH

The programmed number of periods of CLK that refresh

RASs are asserted can be extended by one or multiple peri-

ods of CLK Only the all RAS (with or without error scrub-

bing) type of refresh can be extended To extend a refresh

cycle the input extend refresh EXTNDRF must be assert-

ed before the positive edge of CLK that would have negated

all the RAS outputs during the refresh cycle and after the

positive edge of CLK which starts all RAS outputs during the

refresh as shown in

Figure 13 This will extend the refresh to

the next positive edge of CLK and EXTNDRF will be sam-

pled again The refresh cycle will continue until EXTNDRF is

sampled low on a positive edge of CLK

44 CLEARING THE REFRESH ADDRESS COUNTER

The refresh address counter can be cleared by asserting

RFSH while DISRFSH is negated as shown in

Figure 14a

This can be used prior to a burst refresh of the entire memo-

ry array By asserting RFSH one period of CLK before

DISRFSH is asserted and then keeping both inputs assert-

ed the DP8520A21A22A will clear the refresh address

counter and then perform refresh cycles separated by the

programmed value of precharge as shown in

Figure 14b An

end-of-count signal can be generated from the Q VRAM

address outputs of the DP8520A21A22A and used to ne-

gate RFSH

45 CLEARING THE REFRESH REQUEST CLOCK

The refresh request clock can be cleared by negating

DISRFSH and asserting RFSH for 500 ns one period of the

internal 2 MHz clock as shown in

Figure 15 By clearing the

refresh request clock the user is guaranteed that an inter-

nal refresh request will not be generated for approximately

15 ms one refresh clock period from the time RFSH is neg-

ated This action will also clear the refresh address counter

TLF9338 – 19

FIGURE 13 Extending Refresh with the Extend Refresh (EXTNDRF) Input

TLF9338 – 20

FIGURE 14a Clearing the Refresh Address Counter

TLF9338 – 21

FIGURE 14b Clearing the Refresh Counter during Burst

TLF9338 – 22

FIGURE 15 Clearing the Refresh Request Clock Counter

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