Date: Thu, 28 Nov 2002 10:11:28 -0700 From: Bruce Johnson <[EMAIL PROTECTED]>
On Thursday, November 28, 2002, at 09:38 AM, Rik Gresham wrote:
It's not just your opinion; it's Motorola's opinion too. :-)I have both of these but I am running the Sonnett as it runs cooler than the XLR8, though the latter never seemed to exhibit any strange behavior running 20 degrees hotter than the former...How did you measure these temps? My personal feeling is that any method other than putting a thermocouple on the processor is just fiction.
See, e.g., an1800.pdf which states:
1.2.4 TAU Calibration
Uncalibrated, the accuracy of the TAU will vary from part-to-part with process and temperature. Calibrated, the accuracy will more closely approach the resolution of the TAU (see the MPC750 Hardware Speci*cation).
Bits 0*17 of THRM3 are de*ned as �reserved for future use� in the user's manual. However, bits 2*6 can be used to calibrate the TAU and are de*ned in Table 1. For an uncalibrated TAU, these bits should be cleared to 0 to indicate no adjustments made to the thermal sensor. To calibrate the TAU, these bits are set to adjust the thermal sensor so that the TAU reading matches the junction temperature as determined by some other, more accurate, means. For example, an external thermocouple which has good thermal contact with the back of the die will provide an accurate junction temperature measurement if the die temperature is stable. Another approach is to adjust the thermal sensor so that the TAU reading is correct when the junction temperature is controlled by an external device, for example, a thermoelectric cooling plate thermally attached to the back of the die and controlled via a thermocouple fed back to an adjustable current source.
One other potential approach to calibration relies on the comparatively slow rate of change of junction temperature with time. During factory test and laboratory experiments, the junction temperature has been observed to rise from ambient temperature at a peak rate of approximately 1�C per second. Therefore, a calibration value taken as soon as possible after power-up by comparing the TAU sensor reading to ambient conditions (for example, room temperature) could improve accuracy over an uncalibrated TAU. Note that each processor may have a different adjustment required and that once the calibration setting is determined, the value for bits 2*6 should be used thereafter for that part. The programs described herein use the convention that if TAU_Cal_Value[31] = 1 the calibration value is valid and should be used unless changed through recalibration.
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