Power-On; Power-Off - HP PageWriter XL M1705B Service Manual

Power-on and Power-off
When the cardiograph is turned on or off, it follows a sequence of
Sequences
events in applying power to or removing power from its circuits. The
gate array that controls dynamic RAM (DRAM) also controls this
sequence because the DRAM gate array remains powered up even
when the cardiograph is in Standby.
Power-on
When the (On-Standbd key is pressed to power up the cardiograph, the
DRAM gate array asserts the processor reset signal and sends the
UNITON signal to the +5 V regulator to turn it on. When +5 V is
in regulation, the power supply signals the DRAM gate array, which
releases the processor reset.
When the reset is released, the processor begins execution at its
reset vector, FFFFFOh. This location is in ROM, which contains
initialization routines and tables. The flow chart in Figure 5-l shows
the sequence that the CPU executes to initialize the cardiograph and
begin cardiograph operation. Cardiograph operation begins after the
CPU has begun executing the software stored in RAM.
Power-off
The power-off sequence is software controlled. The power-off
sequence is initiated when the [on-Standby) key is pressed, battery
timeout occurs, or the battery is too low to continue. At power-off,
the software sees a keycode and begins housekeeping tasks such
as completing disk I/O requests, turning off the printer, disabling
the keyboard, turning off the keyboard display, and turning off the
preview display if present. When the housekeeping is complete,
the software instructs the DRAM and FGA gate arrays to stop the
processor and shut down the power supply. The details of how the
power supply shuts tlovm are described later in this chapter in the
power supply description.
Another method of shutting dowl the power supply is provided by
the watchdog timer. The watchdog timer, part of the DR.AM gate
array, shuts down the power supply if the CPU is disrupted beyond
recovery and fails to reset the timer before a specified time elapses.
The timer is programmable and can be set by software for two, four,
six, or eight seconds; the time depends on the operation currently
being executed.
5-2 Theory of Operation