AN2323 Freescale Semiconductor / Motorola, AN2323 Datasheet - Page 3

• Manufacturer Part Number: AN2323
• Description: PowerPC MPC7455 I/O Power Evaluation
• Manufacturer: Freescale Semiconductor / Motorola

some additional current from the Tundra Tsi107™ PowerPC host bridge and buffer. This additional current
is very small and does not dramatically effect the measurements.
2.1
Using the technique pictured in Figure 2-2 while running the JPEG compression benchmark, the following
measurement was attained:
The measurement was taken with the caches disabled in order to force all signals to be driven on the bus; a
lower power measurement could be attained with the caches were enabled. Although still very low, this
result conflicts with the initial theoretical power calculation of 17.8 mW.
2.2
Quiescent and leakage currents, pictured in Figures 2-3 account for some power losses and can help explain
the discrepancy between measured and initial theoretical power values. Using the measurement schematic
in Figure 2-2, these power losses can be measured.
For this measurement, a routine was written to continuously branch to itself. With the caches enabled, it is
assumed that the bus will be idle, meaning any current measured can be attributed to power losses. Once
MOTOROLA
Actual Power Measurement Results
Actual vs. Initial Theoretical Power Discrepancies
I = 81.6 mA -> P = V * I and at 2.5 V
Figure 2-3. Quiescent and Leakage Current Diagrams
Freescale Semiconductor, Inc.
Figure 2-2. Actual Measurement Schematic
PowerPC™ MPC7455 I/O Power Evaluation
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Go to: www.freescale.com
P = 204 mW
Actual Power Measurement Results
Tsi107
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