EXTERNAL DEVICE CHARACTERISTICS ADDRESSING

As stated earlier, the 12 address bits required to decode one of the 4096 cell locations within the MK 4027 are multiplexed onto the 6 address inputs and latched into the on-chip address latches by externally applying two negative-going, TTL-level clocks. The first clock, the Row Address Strobe (RAS),latches the 6 row address bits into the chip. The second clock, the Column Address Strobe (CAS), subse-quently latches the 6 column address bits plus Chip Select (CS) into the chip. Each of these clock signals,

~ and

CAS,

triggers off a sequence of events which are controlled by different delayed internal clocks.

Th~ two clock chains are linked together logically in such a way that the address multiplexing operation is done outside of the critical path timing s~nce for read data access. The later events in the CAS clock sequence are inhibited until ~occurence of a delay-ed sign~rived from the RAS clock chain. This

"gated CAS" features allows the

CAS

clock to be ex-ternally activated as soon as the Row Address Hold Time specification (tRAH) has been satisfied and the 6 address inputs have been changed from Row ad-dress to Column adad-dress information. This results in a system limit of tRCD

=

tRAH + tT + tASC (tT

=

MK 4027, to be comparable in performance (access time) with non-multiplexed devices such as the 18-and 22-pin 4K RAMs. In essence, it allows the designer to compensate for system timing skews that may be encountered in the multiplexing operation when addressing the device. In the MK 4027, the

"window" available for multiplexing from row ad-dress to column adad-dress information while still memory cycles. Selection of the various cycles, whe-ther read, write or some combination whe-thereof, is

Data is retrieved from the memory in a read-only cycle by maintaining WRITE in the inactive or high state throughout the portion of the memory cycle in which CAS is active. Data read from the selected cell write cycle" the data input set-up and hold times are referenced to the negative edge of WR ITE rather than to CAS. Note that delaying WR ITE until after the negative edge of

CAS

is termed a "read-write cycle"

rather than read-modify-write. In a read-write cycle, it is not necessary to wait until data is valid at the out-put before the write operation is started. This feature Is very useful when the MK 4027 is used in sequen-tial memory applications or in systems that employ

"interleaving techniques." However, if a true read-modify-write cycle is required (where the write opera-tion occurs after read access), then WR ITE can occur while FfAS and CAS are still active and after tCAC.

To take full advantage of this CAS/WR ITE signal relationship it is necessary for one to understand how the Data Out Latch is controlled. The most import-ant fact to remember is that any change in the condi-tion of the Data Out Latch is initiated by the CAS ne-gative edge. The output buffer is not affected by

memory cycles in which only the RAS signal is ap-plied to the MK 4027. Whenever CAS makes a negative transition, the output will go unconditionally open-circu ited, independent of the state of any other input to the chip. If the cycle in progress is a read, read-modify-write, or a delayed write cycle and the chip is selected, then the output latch and buffer will again go active, and at access time will contain the data read from the selected cell. This output data is the same polarity (not inverted) as the input data. If the cycle in...2l9gress is a write cycle (WR ITE active low before CAS goes low) and the chip is selected, then at access time the output latch and buffer will contain the input data. Once having gone active, the output will remain valid until the MK 4027 receives the next

CAS n~ive edge. InterveniniLEfresh cycles in which RAS is received, but no CAS, will not cause valid data to be affected. Conversely, the output will assume the open-circuited state during any~le in which the MK 4027 receives a CAS but no RAS sig-nal (regardless of the state of any other inputs). The output will also assume the open-circuit state in nor-mal cycles if the chiQjs unselected. Note that if the chip is unselected (CS high at CAS time) WR ITE commands are not executed and, consequently, data stored in the memory is unaffected.

The three-state data output buffer presents the data output pin with a low impedance to VCC for a logic 1 and a low impedance to VSS (Ground) for a logic O. The effective resistance to VCC (logic "1"

state) is 420.12 maximum and

<

100 n typically. The resistance to VSS (logic "0" state) is 125.12 maximum and

<

50ntypically. The separate VCC pin allows the output buffer to be powered from the positive supply voltage of the logic to which the chip is inter-faced. During battery standby operation, the VCC pin may have power removed without affecting the MK 4027 refreshJ~ill1ration. This allows all system logic except the RAS timing circuitry and the refresh address logic to be turned off during battery standby to conserve power.

Specified on the MK 4027 data sheet are two electrical characteristics of the device which guaran-tee the appropriate state of the data 0R)t'ut during a write cycle. These two specifications, S to WR ITE delay (t RWD) and CAS to WR ITE delay (tCWD) are not restrictive operating parameters. They are includ-ed in the data sheet as electrical characteristics only.

The values listed in the "minimum" and "maximum"

columns should be inserted as terms in the following equations:

1. If tCWD + tT ,,;; tCWD (min), the data out latch will contain the data written into the se-lected cell.

2. If tCWD;;' tCWD (max) + tT and tRWD ;;.

tRWD (max)

+

tT, the data out latch will con-tain the data read from the selected cell.

3. If tCWD does not meet the above constraints then the data out latch will contain indetermi-nate data at access time.

The following diagrams are representations of the MK 4027 timing waveforms for read, write and delay-ed-write or read-modify-write cycles. A list of the timing parameters associated with each cycle is also included.

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These parameters apply to all MK 4027 memory cycles:

SYMBOL tRFSH tRP tRCD

tASR tRAH tASC tCAH tAR

tcsc

tCH tCHR tCRP tOFF tRAS tCAS tRAC tCAC tT

DEFINITION

Maximum time that the device will retain stored data without being refreshed.

RAS

precharge, or RAS inactive time of a cycle.

RAS to

CAS

lead time. Operation within the tRCD (max) limit insures that tRAC (max) can be met. tRCD (max) is specified as a reference point only; if tRCD is greater than the specified tRCD (max) limit, then access time is controlled exclusively by tCAC.

Row address set-up time.

Row address hold time.

Column address set-up time.

Column address hold time.

Column address hold time referenced to RAS.

Chip select set-up time.

Chip select hold time.

Chip select hold time referenced to RAS.

CAS

inactive to

RAS

active precharge time.

Output buffer turn-off delay.

~ pulse width or active time.

CAS

pulse width or active time.

Access time from RAS falling edge.

Access time from

CAS

falling edge.

Transition time (rise and fall). Transition times are measured between VIHC or VIH and VIL. VIHC (min) or VIH (min) and VIL (max) are reference levels for measuring timing of Input signals.

V-8

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