I/O Subsystem
R
204
Intel
®
855PM Chipset Platform Design Guide
9.7. FWH
The following provides general guidelines for compatibility and design recommendations for supporting
the FWH device. The majority of the changes will be incorporated in the BIOS. Refer to the
Intel
®
82802AB/82802AC Firmware Hub (FWH) Datasheet
or equivalent.
9.7.1. FWH
Decoupling
A 0.1-µF capacitor should be placed between the V
CC
supply pins and the V
SS
ground pins to decouple
high frequency noise, which may affect the programmability of the device. Additionally, a 4.7-µF
capacitor should be placed between the V
CC
supply pins and the V
SS
ground pins to decouple low
frequency noise. The capacitors should be placed no further than 390 mils from the V
CC
supply pins.
9.7.2.
In Circuit FWH Programming
All cycles destined for the FWH will appear on PCI. The Intel 82801DBM ICH4-M hub interface to PCI
Bridge will put all CPU boot cycles out on PCI (before sending them out on the FWH interface). If the
ICH4-M is set for subtractive decode, these boot cycles can be accepted by a positive decode agent on
the PCI bus. This enables the ability to boot from a PCI card that positively decodes these memory
cycles. In order to boot from a PCI card, it is necessary to keep the ICH4-M in subtractive decode mode.
If a PCI boot card is inserted and the ICH4-M is programmed for positive decode, there will be two
devices positively decoding the same cycle.
9.7.3.
FWH INIT# Voltage Compatibility
The FWH INIT# signal trip points need to be considered because they are NOT consistent among
different FWH manufacturers. The INIT# signal is active low. Therefore, the inactive state of the Intel
82801DBM ICH4-M INIT# signal needs to be at a value slightly higher than the V
IH
min FWH INIT#
pin specification. The inactive state of this signal is typically governed by the formula V_CPU_IO(min)
– noise margin. Therefore, if the V_CPU_IO(min) of the processor is 1.60 V, the noise margin is 200
mV and the V
IH
min spec of the FWH INIT# input signal is 1.35 V, there would be no compatibility
issue because 1.6 V – 0.2 V = 1.40 V which is greater than the 1.35 V minimum of the FWH. If the V
IH
min of the FWH was 1.45 V, then there would be an incompatibility and logic translation would need to
be used. The examples above do not take into account any noise that may be encountered on the INIT#
signal. Care must be taken to ensure that the V
IH
min specification is met with ample noise margin. In
applications where it is necessary to use translation logic, refer to Section 4.1.4.1.7.
The solution assumes that level translation is necessary. Figure 23 in Section 4.1.4.1.7 implements a
solution for the ICH4-M FWH signal INIT#. Trace lengths and resistor values can be found in Table 14.
The Voltage Translator circuitry is shown in Figure 24. It is strongly recommended that any system that
implements a FWH should have its INIT# input connected to the ICH4-M.
