82443MX Datasheet, PDF (83/173 Pages) Intel Corporation – PCIset

82443MX PCIset
7.2.7 DRAM POWER THROTTLING
7.2.7.1 Overview
The 440MX provides a mechanism to control the rate of write and read cycles to DRAM. This mechanism,
known as “Power Throttling,” includes two independent hardware functions, one for “write throttling” and the
other for “read throttling.”
Write throttling is provided to avoid over-heating when the system generates write traffic to DRAM in a higher
rate than specified for a sustained period of time. A high write rate can fail the system and cause permanent
damage to the 440MX.
The read throttling is provided to avoid over-heating the SDRAM components when the system generates
read traffic to DRAM in a higher rate than specified, for a sustained period of time. Such high read rates can
fail the system.
7.2.7.2 Conceptual Description of Power Throttling
Since read and write throttling operate similarly, the power throttling sequences described in Sections
7.2.7.2.1 and 7.2.7.2.2 apply to both functions.
7.2.7.2.1 DURING MONITORING REGIME

A “global sampling window” is used as a period of monitoring the DRAM traffic.

During the sampling window, the number of transferred Qword writes (for write throttling) and reads (for
read throttling) are counted.

At the end of the sampling window, the accumulated Qword count is compared to a “Global QW
Threshold.” If the count is higher than the threshold, then a “Throttling Regime” is entered; otherwise,
the sequence repeats itself without performing throttling.

The threshold is selected based on the system’s Bandwidth Threshold that guarantees that the 440MX
temperature remains under the specified value. For example, if the sampling window is 1 second, the
maximum sustained bandwidth is 500 MB/s than the desired threshold is 500/8 = 62.5M QW.

The choice of sampling window is made based on the temperature gradient, resulted by a change of
bandwidth from some steady state to the maximum bandwidth. The sampling window of choice is
expected to be a fraction of and up to one second.

Note that during the monitoring regime, no throttling occurs. Only the transfers are counted.
7.2.7.2.2 DURING THROTTLING REGIME

A “throttling time” is selected for the throttling regime duration. The throttling time can be set to an
integer of 1 to 63 times the global sampling window.

The throttling time is selected in a manner that optimizes the relationship between the throttling factor
and the throttling duration. To achieve a given “average bandwidth” that guarantees the desired
component temperature, throttling can be “deeper” for a shorter time or not as deep for a longer time.
For example, assuming a maximum bandwidth of 800 MB/s and a desired sustained bandwidth of 500
MB/s, if the sampling period is set to one second, then the 500 MB/s can be achieved by throttling for
one second down to 200 MB/s, or by throttling for two seconds at 350 MB/s.

The throttling time is further divided into sub-periods called “throttle monitoring windows.” While a typical
throttling time is one or more seconds, each throttle monitoring window period is typically 10
microseconds.
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