CPU
P
ROCESSOR
P
OWER
M
ANAGEMENT
A form of power management called "throttling" is supported on the EPM-CPU-10. This is an
Intel 440BX chipset feature that has been augmented with an I/O control bit in the VersaLogic
Special Control Register. CMOS Setup options have been implemented to select throttling
percentages from 12.5% to 75%, and to enable or disable throttling. Throttling works by
activating the CPU Stop-Clock line every 250 microseconds to create a duty-cycle relative to the
selected throttling percentage. These throttling percentages refer to the relative time the CPU is in
a stopped mode.
If throttling is enabled with the percentage set to 75%, the CPU will run at full speed (566 MHz
for the EPM-CPU-10b, EPM-CPU-10c or EPM-CPU-10e and 850MHz for the EPM-CPU-10d)
for 62.5 microseconds (25%) and will be off for 187.5 microseconds (75%) every 250
microsecond period.
Once the throttling percentage is initialized in the CMOS Setup, it can be enabled and disabled by
writing to the “Throttle” control bit in the VersaLogic Special Control Register. See page 41. This
gives the user a very simple means to throttle back during a time of little activity, and to re-
establish full power when needed.
The EPM-CPU-10b Low Power Fanless and the EPM-CPU-10e versions are 566 MHz Celerons
with throttling always enabled at a minimum of 37.5% (which gives an effective CPU speed of
350 MHz). It can be slowed down to the maximum throttling rate of 75%, but not less than
37.5%. The Throttle control bit in the VersaLogic Special Control Register can not be changed on
the EPM-CPU-10b and EPM-CPU-10e. Contact the factory for information on how to change the
throttling percentage via indexed PCI based registers in the chipset.
Typical Power vs Throttling
10
12.5
15
17.5
20
22.5
25
27.5
30
0.0%
12.5%
25.0%
37.5%
50.0%
62.5%
75.0%
Throttling
Typical Power (Watts)
850 MHz
Pentium III
566 MHz
Celeron
Figure 7
EPM-CPU-10 Reference Manual
Reference – 21