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M27W256 Device description
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devices in the array and connected to the READ line from the system control bus. This
ensures that all deselected memory devices are in their low power standby mode and hat
the output pins are only active when data is desired from a particular memory device.
2.4 System considerations
The power switching characteristics of Advance CMOS EPROMs require careful decoupling
of the devices. The supply current, I
CC
, has three segments that are of interest to the system
designer: the standby current level, the active current level, and transient current peaks that
are produced by the falling and rising edges of E
. The magnitude of this transient current
peaks is dependent on the capacitive and inductive loading of the device at the output. The
associated transient voltage peaks can be suppressed by complying with the two line output
control and by properly selected decoupling capacitors. It is recommended that a 0.1µF
ceramic capacitor be used on every device between V
CC
and V
SS
. This should be a high
frequency capacitor of low inherent inductance and should be placed as close to the device
as possible. In addition, a 4.7µF bulk electrolytic capacitor should be used between V
CC
and
V
SS
for every eight devices. The bulk capacitor should be located near the power supply
connection point. The purpose of the bulk capacitor is to overcome the voltage drop caused
by the inductive effects of PCB traces.
2.5 Programming
The M27W256 has been designed to be fully compatible with the M27C256B and has the
same electronic signature. As a result the M27W256 can be programmed as the M27C256B
on the same programming equipments applying 12.75V on V
PP
and 6.25V on V
CC
by the
use of the same PRESTO II algorithm. When delivered (and after each erasure for UV
EPROM), all bits of the M27W256 are in the '1' state. Data is introduced by selectively
programming '0's into the desired bit locations. Although only '0's will be programmed, both
'1's and '0's can be present in the data word. The only way to change a '0' to a '1' is by die
exposure to ultraviolet light (UV EPROM). The M27W256 is in the programming mode when
V
PP
input is at 12.75V, G is at V
IH
and E is pulsed to V
IL
. The data to be programmed is
applied to 8 bits in parallel to the data output pins. The levels required for the address and
data inputs are TTL. V
CC
is specified to be 6.25 V ± 0.25 V.
2.6 Presto II programming algorithm
Presto II programming algorithm allows to program the whole array with a guaranteed
margin, in a typical time of 3.5 seconds. Programming with Presto II involves the application
of a sequence of 100µs program pulses to each byte until a correct verify occurs (see
Figure 5). During programming and verify operation, a Margin mode circuit is automatically
activated in order to guarantee that each cell is programmed with enough margin. No
overprogram pulse is applied since the verify in Margin mode at V
CC
much higher than 3.6V
provides necessary margin to each programmed cell.
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