Answers Database

Power estimation for the 9500 family devices


Record #2653

Product Family:  Hardware

Product Line:  9500

Problem Title:
Power estimation for the 9500 family devices


Problem Description:
Keywords: power, low power, macrocell, high performance,
	  current, 9500 power dissipation, Icc

Urgency: standard

General Description:

The value of the operating current (Icc) calculated from the
equation in the databook for a 9500 device is significantly
different from the actual value measured on the device I/Os.



Solution 1:

The equation states:

Icc (mA) = MChp*(1.7) + MClp*(0.9) + MC(0.006 mA/MHz)f where

Icc = operating current
MChp = Macrocells in high performance mode
MClp = macrocells in low-power mode
MC = total number of macrocells used
f = Clock frequency

The power dissipation in a 9500 device is completely dependent
on the pattern in the device and the state of its I/Os. The
above mentioned equation is only an approximate way of
caluculating the operating current in the device and will not
always work very well. The equation also assumes that the
design has only one master clock operating at a frequency 'f'
and driving all the macrocells that are being used.

For example, if you have a 16 bit counter operating at 50 MHz
in high perfromance mode, then the value for Icc for a 9536
would be as follows :

Icc = 16*(1.7) + 20*(0.9) + 16*(0.006)*50 mA = 50.0 mA.

The same counter operating in low power mode at the same clock
speed would yield an Icc = 37.2 mA.

However, if your design has 2 4 bit counters operating in high
performance mode at 50MHz and 75 MHz respectively, and a block
of combinational logic consisting of 8 macrocells switching at
10 MHz, then

Icc = 16*(1.7) + 20*(0.9) + 4*(0.006)*50 + 4*(0.006)*75 +
8*(0.006)*10 mA = 48.68 mA.

The same design operating in low power mode at the same clock
speed would give an Icc = 35.88 mA.

Looking at the examples mentioned above, it is very clear that
two different designs using the same number of macrocells have
a different value of operating current (Icc). The equation for
the operating current is approximate and will give you a value
that is approximately +/- 20% of the actual Icc.

The power consumed by the 9500 device is also dependent on the
state of its I/O pins. The power consumed by a device whose
I/Os are driving high would be much less than a device whose
I/Os are driving low (sinking current).



End of Record #2653