9.3 ERASING THE EPROM
The
EPROM
can
be
erased by exposure
to
high-intensity ultraviolet (UV) light with a wavelength
of
2537
angstroms. The recommended integrated dose (UV intensity x exposure time)
is
25
Ws/cm
2
.
The
lamps should
be
used
without
shortwave filters and the
EPROM
should
be
positioned about
one inch from the UV tubes.
Ultraviolet erasure clears all bits
of
the
EPROM
to the zero state. Data
is
the entered by programming ones into the desired bit locations.
CAUTION
Be
sure that the
EPROM
window
is
shielded from light except when erasing. This pro-
tects both the
EPROM
and light-sensitive nodes.
9.4 PROGRAMMING FIRMWARE
The MC68705R3, MC68705U3, MC68705R5, and MC68705U5 have
120
bytes
of
mask
ROM
con-
taining a bootstrap program which
can
be
used to program the EPROM. The vector at addresses
$FF6
and
$FF7
is
used to start executing the program. This vector
is
fetched when VIHTP
is
applied
to pin 8 (TIMER pin) and the
RESET
pin
is
allowed to rise above
VIRES
+.
Figure
9-1
provides a
schematic diagram
of
a circuit and a summary
of
programming steps which
can
be
used
to
program
the
EPROM.
It
is
possible to program the
EPROM
via
the bootstrap software and validate its con-
tents
in
the secure mode. The only way to go from the secure mode to the non-secure mode
is
by
erasing the entire
EPROM.
9.5 PROGRAMMING STEPS
The MCM2532 UV
EPROM
must first
be
programmed
with
an
exact duplicate
of
the information
that
is
to
be
transferred to the MC68705R3, MC68705U3, MC68705R5, or MC68705U5. Non-
EPROM
addresses are ignored by the bootstrap. Since the MC68705R3, MC68705U3, MC68705R5,
or MC68705U5 and the MCM2532
are
to
be
inserted and removed from the circuit, they should
be
mounted in sockets.
In
addition, the precaution below must
be
observed (refer to Figure 9-1).
CAUTION
Be
sure
S1
and
S2
are closed and
VCC
and +
26
V
are
not applied when inserting the
MC68705R3, MC68705U3, MC68705R5, or MC68705U5 and MCM2532 into their respec-
tive sockets. This ensures that
RESET
is
held
low
while inserting the devices.
When ready to program the
MC68705R3, MC68705U3, MC68705R5, or MC68705U5 it
is
only
necessary to provide
VCC
and
+26
volts, open switch
S2
(to apply Vpp and VIHTP), and then open
S1
(to remove reset). Once the voltages
are
applied and both
S2
and
S1
are open, the
CLEAR
out-
put
control line
(PB4)
goes high and then low, then the 12-bit counter (MC14040B)
is
clocked by the
PB3
output (COUNT), The counter selects the MCM2532
EPROM
byte which
is
to load the
equivalent
EPROM
byte selected by the bootstrap program. Once the
EPROM
location
is
loaded,
COUNT
clocks the counter to the next
EPROM
location. This continues until the
EPROM
is
com-
pletely programmed at which time the programmed indicator
LED
is
lit. The counter
is
cleared and
the
loop
is
repeated to verify the programmed data. The verified indicator
LED
lights if the program-
ming
is
correct.
9-4
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