56F8167 datasheet(165/184 Pages) FREESCALE

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Power Consumption

56F8367 Technical Data, Rev. 7.0

Freescale Semiconductor

165

Preliminary

C, the internal [dynamic component], is classic C*V2*F CMOS power dissipation corresponding to the

56800E core and standard cell logic.

D, the external [dynamic component], reflects power dissipated on-chip as a result of capacitive loading

on the external pins of the chip. This is also commonly described as C*V2*F, although simulations on two

of the IO cell types used on the device reveal that the power-versus-load curve does have a non-zero

Y-intercept.

Power due to capacitive loading on output pins is (first order) a function of the capacitive load and

frequency at which the outputs change. Table 10-20 provides coefficients for calculating power dissipated

in the IO cells as a function of capacitive load. In these cases:

TotalPower =

Σ((Intercept +Slope*Cload)*frequency/10MHz)

where:

•

Summation is performed over all output pins with capacitive loads

•

TotalPower is expressed in mW

•

Cload is expressed in pF

Because of the low duty cycle on most device pins, power dissipation due to capacitive loads was found

to be fairly low when averaged over a period of time. The one possible exception to this is if the chip is

using the external address and data buses at a rate approaching the maximum system rate. In this case,

power from these buses can be significant.

E, the external [static component], reflects the effects of placing resistive loads on the outputs of the

device. Sum the total of all V2/R or IV to arrive at the resistive load contribution to power. Assume V =

0.5 for the purposes of these rough calculations. For instance, if there is a total of 8 PWM outputs driving

10mA into LEDs, then P = 8*.5*.01 = 40mW.

In previous discussions, power consumption due to parasitics associated with pure input pins is ignored,

as it is assumed to be negligible.

Table 10-25 I/O Loading Coefficients at 10MHz

Intercept

Slope

PDU08DGZ_ME

1.3

0.11mW / pF

PDU04DGZ_ME

1.15mW

0.11mW / pF

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