AN2323 Freescale Semiconductor / Motorola, AN2323 Datasheet - Page 2

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

Evaluating Signal Transitions
Evaluating Signal Transitions
1.2
To evaluate signal transitions, a perl script was written to retrieve data line by line from the logic analyzer
listings, count the number of transitions per signal, and write that number to a file. Also, using the number
of transitions per signal, theoretical power was calculated using the power equation as shown in Figure 1-1.
It is important to note that for the theoretical calculations, voltage swing (2.5 volts) and bus frequency can
be measured and verified; however, for capacitance, a value of 10 pF per pin is used which is derived from
the 8.0 pF maximum value in the hardware specification plus 2.0 pF for connections to the logic analyzer.
1.3
Data was collected over 394,710 clock cycles while running the JPEG decompression benchmark at a bus
frequency of 100 MHz. Average frequency of the 106 active signals was 2.7 MHz, corresponding to an
initial theoretical power value of 17.8 mW.
Taking a closer look at signals and associated average frequencies and power (see Table 1), it is evident that
the low power value can partly be credited to a low frequency of signal transitions.
Part II Actual Power Measurement
Figure 2-2 shows the circuit used to measure actual power for this analysis. In order to measure I/O power
the resistor R74 was removed from an MPC7455 development system and replaced with an ammeter as
shown. Completely isolating OV
2
Evaluating Signal Transitions
Initial Theoretical Results Summary
Address Signals
Data Signals
Control Signals
Table 1. Average Frequency and Power Associated with I/O Signals
Signal
Freescale Semiconductor, Inc.
PowerPC™ MPC7455 I/O Power Evaluation
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Figure 1-1. Power Formula Derivation
is difficult and as Figure 2-2 depicts, any current measurements include
Average Signal Transition
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Frequency (MHz)
0.81
3.71
1.28
Power (mW)
14.84
1.63
1.47
MOTOROLA