Intel
®
E7520 Memory Controller Hub (MCH) Datasheet 229
Functional Description
5.4.6.2 Thermal Management Threshold Calculation
The dissipation on a given DIMM is a combination of endemic dissipation when powered-on, and
dissipation due to access activity (refresh, activates, and read/write commands). The thermal
management threshold is a measure of the amount of activity supported over a fixed period of time
in addition to the “idle” dissipation for that DIMM. To calculate that budget, assumptions must be
made about the maximum allowable case temperature, the inlet air temperature, and the effective
“theta-CA” coefficient for the DRAM devices in the target chassis. (The theta-CA value denotes
degrees at the case per Watt of dissipated power – better cooling yields a lower coefficient, and a
higher budget for activity.)
The idle dissipation for a given DIMM depends upon the number and type of DRAM devices it
carries; i.e: technology, x8 vs. x4 devices, single vs. dual rank, operating frequency, and device
density. The total current drawn by the device can be calculated from DRAM datasheets. Typical
numbers were used to generate the values used in these calculations.
The activity-based dissipation is the total of activates and commands issued over the monitored
period, weighted for their relative power consumption. For most devices, activates burn more
power than read and write commands, and the relative dissipation changes with device density. As
a result, the MCH supports a programmable weight factor for activates vs. commands (DTCL
register, bits 29:28).
The threshold is just a measure of the number of “active” clocks per monitor period supported by
the power budget after idle dissipation is taken into account. The weighting factor for activates vs.
commands allows us to create a fixed “active_current” for each non-idle clock, which in turn
reduces the threshold value to the weighted number of active clocks allowed during the thermal
management monitoring period. The equation to calculate the threshold value then looks like this:
Threshold = (current_allowed – idle_current) • (clocks_per_period) / active_current
The current_allowed parameter is set by max case temperature (85°C), less the inlet temperature
(Ti), divided by the product of theta-CA and the operating voltage. ((85 – Ti)/(theta-CA • 2.5) for
DDR). As an example, if the inlet temperature (at the DIMM, not at the chassis grille) is 40°C, and
the effective theta-CA is 5°/W, the resulting current_allowed per DIMM is 3.6A before case
temperature hits max.
Figure 5-10. Memory Thermal Management Operation
TT = z*GDSW
AC = Activity Count (running total of weighted operations executed)
TT = Thermal Management Time; z*GDSW
GDSW = Global DRAM Sampling Window; y*4ms
GAT = Global Activity Threshold (initiate management if AC > GAT in any GDSW)
TMW = Thermal Managing Monitoring Window; n*16 clks (recommend n*16 = 10 µsec)
TAM = Thermal Managing Activity Max (enforce that AC < TAM in every TMW)
. . .
GDSW GDSWGDSW GDSW GDSW
Increasing time
AC<GAT AC>GAT
TMW TMW TMW
. . .
GDSW
Thermally Managed:
AC < TAM for each TMW
Not Managed
ot Managed
TMW = 10 µsec
