56F8347_07 Datasheet, PDF (155/172 Pages) Freescale Semiconductor, Inc – 16-bit Digital Signal Controllers

Power Consumption
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.
Table 10-25 I/O Loading Coefficients at 10MHz
PDU08DGZ_ME
PDU04DGZ_ME
Intercept
1.3
1.15mW
Slope
0.11mW / pF
0.11mW / pF
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 10-25 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.
56F8347 Technical Data, Rev.11
Freescale Semiconductor
155
Preliminary