User`s manual

BL1600 Power Management s 129
Holdup Time
A few milliseconds of computing time remain until the regulated +5 V
supply falls below ∼4.65 V, even if the power cuts off abruptly. The
amount of time depends on the size of the capacitors in the power supply.
The standard power supply included with Z-Worlds Developers Kit
provides about 10 ms. If the power cable is removed abruptly from the
BL1600 side, then only the capacitors on the board are available, reducing
computing time to a few hundred microseconds. These times can vary
considerably depending on the systems configuration and loads on the 5V
or 9V supplies.
The interval between detection of the power failure detection and entry to
the power-failure interrupt routine is approximately 6 µs or less
Z-World cannot predict how much time will actually be available to save
important state data. The ratio of the raw DC power supplys output
capacitors value to the circuits current draw determines the actual
duration of the holdup time, t
H
.
Multiple Power-Line Insults
This simple setup can fail when multiple power fluctuations follow each
other rapidlya common occurrence. If the BL1600s Z180 microproces-
sor receives multiple /NMIs, it will overwrite an internal register, making a
correct return from the first /NMI impossible. Also, depending on the
number of fluctuations of the raw DC input (and hence, the number of
stacked /NMIs), the microprocessors stack could possibly overflow,
corrupting the programs code or data.
When the Z180 senses an /NMI, it saves the program counter (PC) on its
processor stack. The Z180 copies the maskable interrupt flag, IEF1, to
IEF2 and zeroes IEF1. The Z180 will restore IEF2s saved state informa-
tion when it executes a RETN (return from nonmaskable interrupt)
instruction.