DP8430V datasheet(21/56 Pages) NSC

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50 Refresh Options (Continued)

512 Externally Controlled Refresh Mode

To choose this refresh mode the user must program

ECAS0 e 1 When this mode is selected the user is re-

sponsible for generating refresh requests by asserting the

input RFSH every time a refresh cycle is to be performed In

this refresh mode the output WE functions a RFRQ

When Externally Controlled Refresh is selected the user

may choose to monitor the output ReFresh ReQuest

(RFRQ) for an indication from the DRAM controller that a

refresh is needed When this output asserts it indicates that

the internal refresh clock has expired and that another re-

fresh is necessary Then the user has two options First he

can answer immediately asserting the input RFSH request-

ing a refresh cycle In this case a refresh will take place

immediately if no access is in progress and precharge time

for the previous access has been met See

Figure 14a

TLF11118 – 21

FIGURE 14a Automatic Internal Refresh with Refresh Request (3T of RAS Low during Refresh Programmed)

Second the user may choose not to assert the RFSH input

delaying the refresh until later RFRQ will go high and then

assert (toggle) if additional periods of the internal refresh

clock have expired and the user has not performed a re-

fresh by asserting the input RFSH See

Figure 14b If a time

critical event or a long access like page or static column

mode can not be interrupted RFRQ pulsing high can be

used to increment an external counter This counter can

later be used to perform a burst refresh of the number of

refreshes missed (through the RFSH input) This scheme

can be thought of as Refresh RequestAcknowledge

TLF11118 – 22

FIGURE 14b Refresh Request Timing

DP8430V 数据表(PDF) 21 Page - National Semiconductor (TI)