System Memory Design Guidelines (DDR-SDRAM)
R
164
Intel
®
855PM Chipset Platform Design Guide
6.8.2.
ETS# Design Guidelines
ETS#, as implemented in the Intel 855PM MCH, is an active low signal and does
not
have an integrated
pull-up to maintain a logic ‘1’. As a result of this, a placeholder for an external 8.2-k
to 10-k
pull-up
resistor should be provided near the ETS# pin. Electrical details on output characteristics of suitable
thermal sensors for use with the MCH are currently not finalized. The recommended pull-up voltage for
this external pull-up is 2.5 V (VCCSM). Ideally, the thermal sensor should implement an open drain
type output buffer to drive ETS#. A system is expected to have one thermal sensor per SO-DIMM
connector on the motherboard. As a result, routing guidelines for the output of these thermal sensors to
the ETS# pin will also be important.
Routing guidelines and other special, motherboard design considerations will vary with the vendor and
type of thermal sensor chosen for this ETS# application. As a result, vendor specific design guidelines
should also be followed closely to ensure proper operation of this feature. As a general rule, system
designers should follow good design practices in ensuring good signal integrity on this signal as well as
achieving adequate isolation from adjacent signals. Also, any thermal design considerations (e.g. proper
ground flood placement underneath the external thermal sensor; proper isolation of the differential
signal routing for thermal diode applications, etc.) for the external thermal sensor itself should also be
met.
6.8.3.
Thermal Sensor Placement Guidelines
The many factors that can affect the accuracy of ambient temperature measurements by thermal sensors
make the placement of them a very critical and especially challenging task. Ideally, one thermal sensor
should be placed near each SO-DIMM in a system. The thermal sensor should be located in an area
where the effects of airflow and effects of conduction from adjacent components are minimized. This
allows for the best correlation of thermal sensor temperature to chassis or notebook surface temperature.
Refer to Figure 92 for details.
Assuming airflow is negligible within a system, the optimal placement of the thermal sensor is on the
surface of the motherboard directly beneath the shadow of an SO-DIMM module centered longitudinally
and laterally in relation to the outline of the SO-DIMM. The thermal sensor should have a form factor
small enough to allow it to fit beneath double-sided memory modules (i.e. modules with memory
devices on both sides of a module). If placement within the outline of an SO-DIMM is not possible, then
the next best option is to locate it within approximately 15 mm (0.6 inches) of the outline/SO-DIMM
shadow. Again, this assumes negligible effects from airflow.
