Elektor EPROM Manual Download Page 5

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Blank mode

This mode actually has two functions: mimic
an empty (blank) EPROM and clearing the
RAM IC. The signals used during Blank mode
are shown in

Figure 3

. Most EPROM pro-

grammers will check if the device is empty
before programming it. The conclusion that
the entire EPROM is empty is also employed
to speed up the programming process.
Because the programmer ‘knows’ that every
address reads FF, it may safely skip all
addresses that have to contain FF. This cuts
down on programming time, although it does
require the EPROM emulator to be ‘empty’
before it is programmed.

The normal procedure to check if an

EPROM is empty is to read all addresses and
compare the resultant data against the value
FF. To do so, your programmer will generate
all possible addresses and generate a read
pulse at each of them. These pulses may be
used to clear the EPROM emulator memory.
Each address is programmed by converting
the read pulse from the programmer into a
write pulse for the RAM IC. All datalines
being pulled to +5 V via resistor array R9,
IC3 is then filled with the value FF at all
memory locations.

At this point, the programmer has issued

a read command and expects device data
(preferably FF) to be placed on to the data-
bus. This is arranged by buffer IC5. The DIR
inputs remain Low, so the A signals are con-
figured as outputs and the B signals, as
inputs. The B signals, too, are connected to
+5 V via resistor array R9, so the buffer IC
will faithfully place the desired data (FF) on
to the databus.

Program mode

The GAL monitors the signal EPV

to detect

the presence of a programming voltage
(

≥

12.5 mming mode. An EPROM being pro-

grammed responds differently to the signals
at CS and OE. Actually, in this state, these
signals may be renamed to WR and RD. Also,
there is no longer an additional signal avail-
able to enable the EPROM to be selected. For-
tunately, that is not necessary during pro-
gramming, because there is no need for the
EPROM to share the data bus and address
bus with other memory devices. In this state,
therefore, it appears as if the EPROM is per-
manently selected — see also

Figure 4

During a programming pulse, the buffer IC

has to transfer the data from the programmer
to the RAM (IC3). This is achieved by pulling
the DIR signal logic High. During a read pulse
(OE activated), the HC245 device has to
buffer the data in the opposite direction. This

EPROM and the databus. This IC is
necessary while the emulator is in
‘Blank’ mode when it is exptected to
supply the value FF at all times dur-
ing a read command. The GAL
arranges for the driver to be acti-
vated in the direction B

A and the

EPROM is not read (actually, a write
command is given to the RAM IC,
more about this further on). Because
of the pull-up resistors inside R9, IC5
sees only logic Ones at the B inputs,
and faithfully copies these to the
external databus.

Some EPROM programmers are

equipped with pull-down resistors.
That is why IC5 is included in the
circuit, since without it the resis-
tance value of R9, depending on the
programmer, would have to be
selected low enough to prevent the
emulator acting as a significant (but
unwanted) voltage source in the tar-
get circuit.

Switch S1 allows you to switch

the EPROM emulator into Blank
mode, while S2 needs to be actuated
to manually leave this mode.

Finally, there are the components

around voltage regulator IC1. This is
a standard 5-volt regulator that steps
down the 9-V input voltage to 5
volts required by the logic circuitry.

The GAL

Apart from IC3, the most important
part of the circuit is the GAL chip.
As already mentioned, it contains all
logic circuitry we need to translate
and format external signals so that
they can be understood by the RAM
chip.

The JEDEC source file listing for

the GAL is shown in

Figure 2

Although an uncomplicated and
modest bit of programming, the
equivalent logic function of the GAL
is the same as that offered by a
rather more than a handful of ICs…
which if used would make the pro-
ject unwieldy.

The GAL will operate in one of

three modes: Blank, Normal and Pro-
gram. Each of these modes requires
the GAL to work in a different man-
ner. The Blank and Normal modes
may be selected manually by the
user. By pressing S1 once, the GAL
is configured for

Blank mode

. The

contents of the emulator may then
be erased by means of a blank check
run by the programmer. This state is
held until the user presses S2, or the
programmer starts to program the
emulator.

Normal mode

is selected by

pressing S2 once. The emulator unit
will hen behave like an ordinary
27C256 EPROM.

Program mode

is automatically

selected as soon as the programmer
starts to program the emulator (V

raised to >12.5 volts). Once the pro-
gramming is finished (V

dropped

to +5 V), the emulator automatically
returns to Normal mode.

MICRO

PROCESSOR

11/2002