Texas Instruments 990 Operation Manual page 174

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945255-9701
There must be a delay after each attempt to program a bit string before trying to program the
same bit string again. This delay is necessary to allow the charge to diffuse into the EPROM
device without a buildup of excess charge on the surface.
Because of the required delay, each bit string of the EPROM should be attempted once before
repeating the programming cycle. To ensure this delay, the number of retries for programming
each bit string (defined in the RC subcommand) must be set to zero. Each bit of the EPROM
will not appear to have the correct value (0 or 1) until sufficient charge has been transferred to
it.
In the early stages of programming, the bits may not have acquired sufficient charge to have the
correct value. This will appear as a programming failure if the number of retries is set to a
nonzero value, and the bit string will be programmed again without the required delay time. For
the same reason, the compare toggle (defined by the TS subcommand) should not be set during
the programming cycle, since compare errors will be found in the early stages of programming an
EPROM.
Since the programming cycle for an EPROM repeats many times, the display toggles (defined by
the TS subcommand) should not be set during the programming cycle since the memory or
PROM data will be printed for each repetition.
Therefore to program, compare, and display, the process must be done in two steps. First the
toggles must be set to program. After completion of programmi..'1g the EPROM, the toggles may
be set to compare and/or display. The number of repetitions defined must be changed to one
before the second step to compare and/or display.
The following example shows how to program a 1024 X 8 EPROM from a 1024 word block of
memory. The following commands define the memory and PROM data configurations, bit string
width, PROM characteristics, memory bounds, and toggles, and initiate the programming process.
PP,MI,1,10,400,O
PP,RI,1,8,400,O
PP,M 1,2,O,C8,O
PP,RI,2,O,C8,O
PP,SW,8
PP,RC,8,O,1,O,32,8
PP,MB,O,3FF
PP, TS,O,O, 1,0
PP,GO
The level 2 mapping defines the repetition count to be C8
16
=
200. The toggles are set to program
memory to PROM.
To perform the compare to check for programming failures, the following commands are needed.
PP,MI,2
PP,RI,2
PP,TS,O,O,O,1
PP,GO
The RI and MI subcommands define the repetition count to the default value of 1. The toggles
are set to compare memory to PROM.
7-24
Digital Systems Division .