I/O Subsystem
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Intel
®
855PM Chipset Platform Design Guide
225
at higher frequencies and will degrade the transmit BER performance. Caution should be
exercised if a cap is put in either of these locations. If a cap is used, it should almost certainly be
less than 22 pF. (6 pF to 12 pF values have been used on past designs with reasonably good
success.) These caps are not necessary, unless there is some overshoot in 100 Mbps mode.
Note:
It is important to keep the two traces within a differential pair close
†
to each other. Keeping them close
†
helps to make them more immune to crosstalk and other sources of common-mode noise. This also
means lower emissions (i.e. FCC compliance) from the transmit traces, and better receive BER for the
receive traces.
†
Close should be considered to be less than 0.030 inches between the two traces within a differential
pair. 0.007 inch trace-to-trace spacing is recommended.
9.10. Power
Management
Interface
9.10.1.
SYS_RESET# Usage Model
The System Reset signal (SYS_RESET#) of the Intel 82801DBM ICH4-M can be connected directly to
a reset button or any other equivalent driver in the system where the desired effect is to immediately put
the system into reset. If an Intel® Pentium® M Processor / Intel® Celeron® M Processor ITP700FLEX
debug port is implemented on the system, Intel recommends that the DBR# signal of the ITP interface
be connected to SYS_RESET# as well. If SYS_RESET# is implemented, a weak pull-up resistor pulled-
up to the 3.3-V standby rail (VccSUS3_3) should also be implemented to ensure that no potential
floating inputs to SYS_RESET# cause a system reset. The ICH4-M will debounce signals on this pin
(16 ms) and allow the SMBus to go idle before resetting the system. This delay to allow all outstanding
SMBus cycles to complete first and to prevent a slave device on the SMBus from “hanging” by resetting
in the middle of an SMBus cycle.
9.10.2.
PWRBTN# Usage Model
The Power Button signal (PWRBTN#) of the Intel 82801DBM ICH4-M can be connected directly to a
power button or any other equivalent driver (e.g. power management controller) where the desired effect
is to indicate a system request to go to a sleep state (if in a normal operating mode) or to cause a wake
event (if in a sleep state already). This signal is internally pulled-up in the ICH4-M to the 3.3-V standby
rail (VccSUS3_3) through a weak pull-up resistor (20 k
nominal). The ICH4-M has 16 ms of internal
debounce logic on this pin.
9.10.3.
Power Well Isolation Control Strap Requirements
The RSMRST# signal of the ICH4-M must transition from 20% signal level to 80% signal level and
vice-versa in 50us. Slower transitions may result in excessive droop on the VCCRTC node during Sx-to-
G3 power state transitions (removal of AC power). Droop on this node can potentially cause the CMOS
to be cleared or corrupted, the RTC to loose time after several AC power cycles, or the intruder bit
might assert erroneously.
The circuit shown in the figure below can be implemented to control well isolation between the
VccSUS3_3 and RTC power-wells in the event that RSMRST# is not being actively asserted during the
discharge of the standby rail or does not meet the above rise/fall time
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