Z-World OP7100, Руководство пользователя

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OP7100 26 s Hardware
ADM691 Supervisor Chip
A voltage divider consisting of R29 and R30 across the DC input provides
a
PFI
signal to the ADM691 watchdog supervisor. The ADM691 chip
performs the following services.
• Watchdog timer resets the microprocessor if software “hangs.”
• Power-failure shutdown and reset.
• Generates an “early warning” power-failure interrupt (PFI) that lets the
system know when power is about to fail.
• Memory protection feature prevents writes to RAM when power is low.
• Supports battery backup.
Handling Power Fluctuations
During a normal power-down, an interrupt service routine is used in
response to a nonmaskable interrupt (NMI) to save vital state information
for the application for when power recovers. The amount of code that the
interrupt serive routine can execute depends on the voltage rate of de-
crease.
Theoretically, a power failure would cause a single
NMI.
Then, the inter-
rupt service routine would restore data from the previous state when the
voltage recovers.
However, fluctuations in the
DC
input line could cause the ADM691 to see
multiple crossings of the
1.3 V
input power-reset threshold. These multiple
negative-edge transitions would, in turn, cause the Z180 to see multiple
NMI
s.
When the Z180 generates an NMI, it saves the program counter (PC) on
the processor’s stack. It next copies the maskable interrupt flag, IEF1, to
IEF2 and zeroes IEF1. The Z180 will restore saved state information when
it executes a RETN (return from nonmaskable interrupt) instruction.
Ideally, the Z180 should be able to pop the stack and return to the location
where the program was first interrupted. But the original IEF1 flag is not
recoverable because the second and subsequent NMIs will have saved
IEF1 = 0 to IEF2. Also, depending on the number of fluctuations of the DC
input (and hence, the number of stacked NMIs), the processor’s stack can
overflow, possibly into your program’s code or data.
The following sample program shows how to handle an
NMI
.