Answers Database
PROMS: Can reset polarity be changed on a programmed device by reprogramming it?
Record #1496
Product Family: Hardware
Product Line: 1700D
Product Part: 1700D
Problem Title:
PROMS: Can reset polarity be changed on a programmed device by reprogramming it?
Problem Description:
Urgency: standard
General Description:
Occasionally users may inadvertently program their
1700/1700D/1700L PROMs to the wrong reset polarity, and the
question arises as to whether it is possible to change only
the reset polarity on a programmed device.
Solution 1:
Generally speaking, the XC1700/D/L serial PROMs are one-time
programmable devices from the standpoint of the bulk of the PROM's
data/contents. HOWEVER, the PROM reset polarity
is an exception to this rule, although only in one special
case. The special case is when the reset polarity in the
programmed part has been programmed for active HIGH reset.
In this case, the reset polarity can actually be changed from
Active High to Active Low by reprogramming the part. Note,
however, that the converse is not true--you cannot reprogram
the reset polarity from active LOW to active HIGH.
How is this possible? Consider that the reset polarity of
the PROM is controlled by the data in a specific set of 4
addresses on the PROM. The unprogrammed state of these
special PROM address locations is FF, and you can only change
the individual bits at these locations from a 1 to a 0 during
programming. The reset polarity is active HIGH when the
special set of bytewide address locations is set to FF
(or 1111 1111 in binary), so essentially, in the unprogrammed
state (FF), these address locations by default would set the
PROM to active HIGH reset.
It follows that choosing active HIGH reset polarity during
the programming process is equivalent to leaving these
address locations unprogrammed.
As a result, once the part has been programmed for active
HIGH reset, you can still change the data in these locations
from FF to 00 (i.e., you can still "reprogram" them to 00)
through the programmer software interface, using the same
data file.
End of Record #1496 - Last Modified: 12/27/99 16:44 |
