Little Board™/486e Technical Manual
2–8
Powerfail Monitor
The Little Board/486e CPU has a built-in powerfail circuit that generates a clean reset signal if
power falls below 4.65V. It guarantees a minimum 1 40 mS re set sign al, ind epe nden t of ho w long
the power falls below the 4.65V threshold .
To enable the powerfail circuitry, short W14 with a jumper.
Non-maskable interrupt (NMI): When the supply voltage falls below (approximately) 4.7 volts,
the powerfail logic sends an NMI to the CPU . Whe n the BI OS de tec ts the NM I, it di splay s the
message “Power Fail NMI” on the screen. You have two options at this point (made by keyboard
selections). One is to mask the NMI and continue. The other is to reboot the system. This requires
operator intervention. If you want an automatic response to the NMI, you can provide an NMI
handler in your application, and patch the NMI inte rrupt vec tor ad dre ss to point to yo ur rou tine .
If you have configured the byte-wide socket S0 for battery backup, it will be write protected while
power is below 4.7 volts. (Its chip select is he ld to a logic 1.) Thi s is to pre vent w ritin g bad dat a to
an SRAM in S0 when the voltage is low.
Hardware reset: If the supply voltage falls below ( appro ximate ly) 4 .40V, the po werf ail lo gic
initiates a hardware reset (like pressin g th e RE SET bu tto n). A “cle an” r eset dur ing a low v olt age
period prevents erratic operation or crashes. Reset is asserted for the duration of the low-voltage
period plus 100 mS after the voltage returns to above 4.4 0V.