3.11.4
NMI Timer
The VP7 features a timer which can trigger NMIs periodically. The timer is programmable in 7 steps
from 0.25 ms up to 16 ms. The usage of the NMI timer together with appropriate software available
from your distributor, Windows 3.11, Windows 95 and Windows NT offer "Real Time Functions".
7
6
5
4
3
2
1
0
NMITCR
RES
TSEL2
T_SEL1
T_SEL0
I/O
NMI Timer Control Register
reset: 11111000
$161w
T_SEL[ ]
Enable and disable the NMI-timer and selects the NMI-timer period
T_SEL2..T_SEL0
NMI period
000
NMI-timer disabled
001
0.256 ms
010
0.512 ms
001
1.024 ms
100
2.048 ms
101
4.096 ms
110
8.192 ms
111
16.384 ms
RES
Reserved for future use.
After a NMI has occurred, the external NMI ticker circuitry must first be reset by an IO
write (don’t care value) to a shadow port of port 061h at address 861h, with the lock
register LOCKR unlocked. Only this cycle will pass the PCI to ISA bridge and will reset
the ticker. After this, the IOCH bit of PORT061 has to be cleared by setting and clearing
the ENIOC bit.
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