Octagon PC-510, Руководство пользователя

Страница: 157 / 239

21-2
CPU power management PC-510 user’s manual
In a stand-alone environment (no APM software active), the firmware
works in conjunction with the hardware timers and monitoring func-
tions to identify periods when certain devices or the entire system are
inactive. Individual timers are supported for specific devices, including
the hard disk, floppy disk and serial ports. Whenever these devices are
not accessed for a specified period, they are powered down to reduce
system power consumption. Whenever none of the monitored system
devices has been accessed for a specified period of time, the performance
of the system is reduced or the system is stopped altogether to further
reduce power consumption.
In a cooperative environment, devices are still controlled by the firm-
ware, but the CPU is never slowed or stopped without the consent of the
APM software. Rather, the firmware notifies the software when a timer
has expired or some other event has occurred which should place the
system in a reduced power mode. The APM software polls the firmware
for such events. Once an event has occurred, the software initiates the
reduced power mode by acknowledging the event back to the firmware.
The firmware then initiates the reduced power mode. The APM soft-
ware can inquire “APM aware” applications to ensure that the reduced
power mode is acceptable.
≡
Hardware controlled modes
The firmware assumes that keyboard, disk, and video access should
prevent the system from entering DOZE mode. In addition, specific
interrupts can be configured to reset the DOZE timer. DOZE mode is
typically a reduced clock operation, but a stopped-clock mode is also
supported. If the slow-clock mode is selected, the CPU clock is slowed
from 33 MHz to 8 MHz. If stopped-clock mode is selected, the secondary
DOZE timer is configured to restart the CPU clock whenever the timer
interrupt occurs. This allows the CPU to maintain the system clock
while in stopped-clock mode even though the CPU clock remains
stopped for more than 90% of the time.
The CPU activity monitor (cool-down clocking mechanism) is intended to
prevent thermal run-away in low airflow environments. It essentially
counts the time that the CPU operates in full-speed mode and the time
spent in reduced clock mode. If full-speed mode is maintained for a
period of time that could cause excessive heat build-up, then the clock is
reduced for a cool-down period to maintain an equilibrium level. The
CPU clock is shifted from 33 MHz to 16.5 MHz for a pre-determined
amount of time, back to 33 MHz, and then the CDC cycle begins again.
≡
Device power management
The hard disk, floppy disk, and serial ports are power managed on an
individual basis. For each device, the firmware configures a hardware
timer that is reset each time the device is accessed. When the device
goes unaccessed for the duration of the timer, an SMI is generated to
notify the firmware that the device is inactive. The firmware can then
SUSPEND that device.