Specifications

Hardware Configuration
2–7
programming Flash EPROMs. (There may be a requirement for an external +12 volt supply,
depending on what peripherals you connect to the Little Board system. For instance, +12V is required
for most flat panel backlight power supplies and for an external Ethernet LAN MAU device
connected to the Ethernet controller AUI interface.)
The exact power requirement of the Little Board/486i system depends on several factors, including
the quantity of DRAM, installed byte-wide memory devices, SCSI bus termination, CPU speed, the
peripheral connections, and which, if any, MiniModule products or other expansion boards are
attached to the PC/104 bus. For example, AT keyboards draw their power from the board, and there
can be some loading from the serial and parallel ports. Consult the specifications in Chapter 1 for the
basic power requirements of your model.
If you use a switching power supply, be sure it regulates properly with the load your system draws.
Some switching power supplies do not regulate properly unless they are loaded to some minimum
value. If this is the case with your supply, consult the manufacturer about additional loading, or use
another supply or another type of power source (such as a linear supply, batteries, etc.). The
minimum power for the Little Board/486i appears in the power specifications in Chapter 1.
2.2.2 Powerfail Options
The Little Board/486i includes a circuit that can sense a power failure. If the +5V power supply falls
below 4.7 V, the powerfail logic produces a non-maskable interrupt (NMI). If it falls below 4.5V, it
generates a hard reset.
To enable the powerfail circuitry, short W20 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. When the BIOS detects the NMI, it displays 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, of course, 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 interrupt vector address to point to your routine. See Chapter
3 for additional information about writing an NMI handler for the powerfail interrupt.
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 held to a logic 1.) This is to prevent writing bad data to an
SRAM in S0 when the voltage is low.
Hardware reset: If the supply voltage falls below (approximately) 4.5V, the powerfail logic initiates
a hardware reset (like pressing the RESET button). A “clean” reset during a low voltage 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 4.5V.
2.2.3 Backup Battery
With only the real-time clock drawing current, the backup battery on the Little Board/486i should last
10 years. If it supplies only the clock, replace the battery every 10 years as a routine maintenance
procedure.
If the battery is also used to back up an SRAM in byte-wide socket S0, calculate the battery life using
the formula below. To calculate battery life, add the SRAM current and the clock current (1 uA) and
divide 165 milliamp-hours by the sum. Then, multiply that result by the duty cycle of the battery. That
is, estimate the percentage of time the battery supplies power (while the system is off).