dp8421a National Semiconductor Corporation, dp8421a Datasheet - Page 18

• Manufacturer Part Number: dp8421a
• Description: Microcmos Programmable 256k/1m/4m Dynamic Ram Controller/drivers
• Manufacturer: National Semiconductor Corporation

11b ) In this mode the output WE always functions as
4 0 Port A Access Modes
4 5 2 Address Pipelining
Address pipelining is the overlapping of accesses to differ-
ent banks of DRAM If the majority of successive accesses
are to a different bank the accesses can be overlapped
Because of this overlapping the cycle time of the DRAM
accesses are greatly reduced The DP8420A 21A 22A can
be programmed to allow a new row address to be placed on
the DRAM address bus after the column address hold time
has been met At this time a new access can be initiated
with ADS or ALE depending on the access mode while
AREQ is used to sustain the current access The DP8422A
supports address pipelining for Port A only This mode can-
not be used with page static column or nibble modes of
operations because the DRAM column address is switched
back to the row address after CAS is asserted This mode is
programmed through address bit R8 (see Figures 11a and
RFRQ
FIGURE 11a Non-Address Pipelined Mode
(Continued)
FIGURE 11b Address Pipelined Mode
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During address pipelining in Mode 0 shown in Figure 11c
ALE cannot be pulsed high to start another access until
AREQ has been asserted for the previous access for at
least one period of CLK DTACK if programmed will be
negated once AREQ is negated WAIT if programmed to
insert wait states will be asserted once ALE and CS are
asserted
In Mode 1 shown in Figure 11d ADS can be negated once
AREQ is asserted After meeting the minimum negated
pulse width for ADS ADS can again be asserted to start a
new access DTACK if programmed will be negated once
AREQ is negated WAIT if programmed will be asserted
once ADS is asserted
In either mode with either type of wait programmed the
DP8420A 21A 22A will still delay the access for precharge
if sequential accesses are to the same bank or if a refresh
takes place
TL F 8588 – G0
TL F 8588 – G1