Theory of Operation
POWER-FAIL PROTECTION
The ZT 8809A supports both DC and AC power-fail protection.
Advantages of each, as well as operation of both types of power-fail
protection, are described in this section.
DC Power-Fail
The factory default setting enables the ZT 8809A to detect 5 VDC
and assert a System Reset if power falls below 4.75 VDC. If the
optional 3.9 V battery is installed, battery voltage switches in to
protect those circuits selected by jumpers for battery backup. The
real- time clock and 32 Kbyte static RAM are always protected if the
battery is installed. Jumpers allow additional battery backup of the
RAM sockets and the configurable RAM/EPROM socket (when
configured for RAM).
DC power-fail protection provides the advantages of system integrity
and battery backup. System integrity is improved because the system
is not allowed to operate if DC voltage is less than 4.75 V. System
Reset is held active both on power rising and power falling so the
processor will not try to function when power is invalid and thereby
possibly corrupt valid data in battery-backed devices. DC power-fail
detection is important for battery backup so battery power may be
switched in at the proper time as DC fails.
A problem with using only DC power-fail detection is that if the
processor is in the middle of a write to battery-backed or non-battery-
backed RAM at the time of power-fail, invalid data could be written
to the correct RAM or even to the wrong RAM. Thus even the
battery-backed RAM could be corrupted if the processor is writing as
power fails. For more complete system integrity, AC power-fail
detection has been provided as an option to the ZT 8809A.
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Содержание ZT 8809A
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