I/O Controller Hub 2
R
Intel
®
Pentium
®
4 Processor / Intel
®
850 Chipset Family Platform Design Guide 187
9.14 SPKR Pin Consideration
The effective impedance of the speaker and codec circuitry on the SPKR signal line must be
greater than 50 kΩ. Failure to due so will cause the TCO Timer Reboot function to be erroneously
disabled. SPKR is used as both the output signal to the system speaker and as a functional strap.
The strap function enables or disables the “TCO Timer Reboot function” based on the state of the
SPKR pin on the rising edge of POWEROK. When enabled, the ICH2 sends an SMI# to the
processor upon a TCO timer timeout. The status of this strap is readable via the NO_REBOOT bit
(bit 1, D31: F0, Offset D4h)
. The SPKR signal has a weak integrated pull up resistor (the resistor
is only enabled during boot/reset). Therefore it’s default state when the pin is a “no connect” is a
logical one or enabled. To disable the feature, a jumper can be populated to pull the signal line low
(see figure). The value of the pull-down must be such that the voltage divider caused by the pull
down and integrated pull up resistors will be read as logic low. When the jumper is not populated,
a low can still be read on the signal line if the effective impedance due to the speaker and codec
circuit is equal to or lower than the integrated pull up resistor. It is therefore strongly
recommended that the effective impedance be greater than 50 kΩ and the pull-down resistor be
less than 7.3 kΩ.
Figure 134. SPKR Circuit
Speaker_Circuit
Stuff jumper to
disable time-out
feature.
R < 7.3 k
Ω
Reff > 50 kΩ
Effective Impedence
due to speaker and
codec circuit.
18 k
Ω - 42 kΩ
Integrated Pull-up
3.3V
ICH2
It should be noted that this is not the only solution to this problem. Board designers can also
isolate the load from the SPKR pin until POWEROK is in a stable high state. This would allow a
weak effective load to be implemented.
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