zpsd211r STMicroelectronics, zpsd211r Datasheet - Page 28

• Manufacturer Part Number: zpsd211r
• Description: Low Cost Field Programmable Microcontroller Peripherals
• Manufacturer: STMicroelectronics

16.0
Power
Management
(cont.)
16.5 Composite Frequency of the Input Signals to the PAD Logic
The composite frequency of the input signals to the PADs is calculated by considering all
transitions on any PAD input signal (including the MCU address and control inputs). Once
you have calculated the composite frequency and know the number of product terms used,
you can determine the total AC current consumption of the PAD by using Figure 13 or
Figure 14. From the figures, notice that the DC component (f = 0 MHz) of PAD current is
essentially zero when the turbo feature is disabled, and that the AC component increases
as frequency increases.
When the turbo feature is disabled, the PAD logic can achieve low power consumption by
becoming active briefly, only when inputs change. For standard voltage (non-V) devices, the
PAD logic will stay active for 25 nsec after it detects a transition on any input. If there are
more transitions on any PAD input within the 25 nsec period, these transitions will not add
to power consumption because the PAD logic is already active. This effect helps reduce the
overall composite frequency value. In other words, narrowly spaced groups of transitions on
input signals may count as just one transition when estimating the composite frequency.
Note that the “knee” frequency in Figure 13 is 40 MHz, which means that the PAD will
consume less power only if the composite frequency of all PAD inputs is less than 40 MHz.
When the composite frequency is above 40 MHz, the PAD logic never gets a chance to shut
down (inputs are spaced less than 25 nsec) and no power savings can be achieved.
Figure 14 is for low-voltage devices in which the “knee” frequency is 20 MHz.
Take the following steps to calculate the composite frequency:
1) Determine your highest frequency input for either PAD A or PAD B.
2) Calculate the period of this input and use this period as a basis for determining the
3) Examine the remaining PAD input signals within this base period to determine the
4) Signal transitions that are spaced further than 25 nsec apart count as a distinct transition
5) Count up the number of distinct transitions and divide that into the value of the base
6) The result is the period of the composite frequency. Divide into one to get the composite
Unfortunately, this procedure is complicated and usually not deterministic since different
inputs may be changing in various cycles. Therefore, we recommend you think of the
situation that has the most activity on the inputs to the PLD and use this to calculate the
composite frequency. Then you will have a number that represents your best estimate at
the worst case scenario.
Since this is a complicated process, the following example should help.
Example Composite Frequency Calculation
Suppose you had the following circuit:
composite frequency.
number of distinct transitions.
(50 nsec for low-voltage V devices). Signal transitions spaced closer than 25 nsec count
as the same transition.
period.
frequency value.
(12 MHz
Crystal)
80C31
AD0-AD7
A8-A16
PSEN
ALE
WR
CSI
RD
ZPSD211R
PB
PC
PA
3 Chip-Select Outputs
3 Inputs: Int, Sel, Rdy
Output (LA0 -LA7)
Latched Address
6 MCU I/O Outputs
PSD211R Family
25