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9.04 POWER FAIL DETECTION CIRCUIT
The Power Failure Detector always monitors:
•
The backup battery to warn of a low battery condition, and
•
The +5V power supply to detect when it falls below 4.75V.
If either of the two above conditions occur, the PFO (Power Fail Output signal) goes low.
In the case of the +5V power supply, a reset will result as explained in the previous
section (9.03.2). In the case of the low battery, what happens will depend on the setting of
the W26 jumper and user-defined algorithm.
The W26 jumper, when shorted, will generate a NMI (Non Maskable Interrupt), through
the IOCHK* line, when the PFO goes low. In turn, this NMI may be serviced by an
interrupt handler to locate the source and notify the user or the system.
If the W26 jumper is left open, no NMI is generated, however an algorithm could be used
to detect a low battery condition and respond accordingly.
A low battery condition is detected, in any case, by polling bit 3 at TEKNOR's I/O base
address register (190H, 290H or 390H, depending on W21 jumper setting): If this bit
reads "1", then the battery voltage is good; if it reads "0", then it is in a low condition
(below 2.9V, typical).
There are three ways to set the Power Fail and Battery circuit:
•
Using Internal Battery & Monitoring External Source:
Internal Battery (set by W27 jumper with pins 1-2 shorted) and External Power Fail
Input to Pin 6 of J9 connector (set by W28 jumper with pins 1-2 shorted).
The power detection input can only accept DC voltage. The line is monitored via a
resistor network made up of a user-defined resistor (R127) and a fixed 1KΩ resistor
connected to GND. The junction of these two resistors is connected to the input of the
power fail circuit, which has its threshold set to 1.25V. R127 can be calculated as
follows:
R127 = 1KΩ (VI-1.25)
1.25
Power Management
where VI is input voltage on pin 6
of the power connector.
9-4
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