Enabling The Oscillator Failure Detection Circuitry - Intel 8XC196NT User Manual

USFR
The unerasable PROM (USFR) register contains two bits that disable external fetches of data and
instructions and another that detects a failed oscillator. These bits can be programmed, but cannot be
erased.
WARNING: These bits can be programmed, but can never be erased. Programming these bits makes
dynamic failure analysis impossible. For this reason, devices with programmed UPROM bits cannot
be returned to Intel for failure analysis.
7
— —
Bit Bit
Number Mnemonic
7:4 —
3 DEI
2 DED
1 —
0 OFD
You can verify a UPROM bit to make sure it programmed, but you cannot erase it. For this reason,
Intel cannot test the bits before shipment. However, Intel does test the features that the UPROM
bits enable, so the only undetectable defects are (unlikely) defects within the UPROM cells them-
selves.

15.3.3 Enabling the Oscillator Failure Detection Circuitry

Programming the OFD bit enables circuitry that resets the device when it detects a failed oscilla-
tor. (See "Detecting Oscillator Failure" on page 12-12 for details.) To program this bit, you must
write the correct value to the location shown in Table 15-4, using slave programming mode. Dur-
ing normal operation, you can determine the value of this bit by reading the USFR (Figure 15-1
on page 15-7). In EPROM packages, the OFD bit can be erased.
— —
Reserved; always write as zeros.
Disable External Instruction Fetch
Setting this bit prevents the bus controller from executing external
instruction fetches. Any attempt to load an external address initiates a
reset.
Disable External Data Fetch
Setting this bit prevents the bus controller from executing external data
reads and writes. Any attempt to access data through the bus controller
initiates a reset.
Reserved; always write as zero.
Oscillator Fail Detect
Setting this bit enables the device to detect a failed oscillator and reset
itself. (In EPROM packages, this bit can be erased.)
Figure 15-1. Unerasable PROM (USFR) Register
PROGRAMMING THE NONVOLATILE MEMORY
DEI DED
Function
Address: 1FF6H
Reset State: XXH
0
— OFD
15-7