I/O Subsystem
R
Intel
®
855PM Chipset Platform Design Guide
205
9.7.4. FWH
V
PP
Design Guidelines
The V
PP
pin on the FWH is used for programming the flash cells. The FWH supports V
PP
of 3.3 V or 12
V. If V
PP
is 12 V, the flash cells will program about 50% faster than at 3.3 V. However, the FWH only
supports 12-V V
PP
for 80 hours (3.3 V on Vpp does not affect the life of the device). The 12-V V
PP
would be useful in a programmer environment, which is typically an event that occurs very infrequently
(much less than 80 hours). The V
PP
pin MUST be tied to 3.3 V on the motherboard.
In some instances, it is desirable to program the FWH during assembly with the device soldered down
on the board. In order to decrease programming time it becomes necessary to apply 12 V to the V
PP
pin.
The following circuit will allow testers to put 12 V on the V
PP
pin while keeping this voltage separated
from the 3.3-V plane to which the rest of the power pins are connected. This circuit also allows the
board to operate with 3.3 V on this pin during normal operation.
Figure 111. FWH VPP Isolation Circuitry
1K
FET
12V (From Motherboard)
3.3V
VPP
9.7.5.
FWH INIT# Assertion/Deassertion Timings
Due to the large routing solution space and necessity of a voltage translator in the design of a FWH on
Intel® Pentium® M Processor / Intel® Celeron® M Processor and Intel 82801DBM ICH4-M based
platforms, the following timing requirements must be met to ensure proper system operation.
For INIT# assertion timings, a conservative analysis of the worst case signal propagation times shows
that no timing concerns exist because the ICH4-M asserts INIT# for 16 PCI clocks (485 ns) before
deasserting. This provides adequate time for INIT# to propagate to both the processor and FWH.
For the INIT# deassertion event, the critical timing is the minimum period of time before the processor
is ready to begin fetching code from the FWH after the INIT# based reset begins. This minimum period
is conservatively set at 1 CPU clock (10 ns). This also represents the maximum allowed propagation
time for the INIT# signal from the ICH4-M to the FWH.
Systems that use alternative devices (i.e. not a FWH) to store the firmware may or may not require the
use of INIT#. If INIT# is not used, an analysis should be done to ensure there is no negative impact to
system operation. If INIT# is implemented on such a device, voltage translation may be necessary, and
the assertion/deassertion timings noted above still apply.
