Datasheet

CY8C21x34/B CapSense
®
Design Guide Document No. 001-66271 Rev. *B 41
The average power consumed by the device can be calculated as follows:
Equation 12
Where:
V
DD
= supply voltage
I
AVE
= average current
An Excel-based average power consumption calculator is available. Click here to download the calculator.
5.5.3 Response Time versus Power Consumption
As illustrated in Equation 12, the average power consumption can be reduced by decreasing I
AVE
or V
DD
. I
AVE
may be
decreased by increasing sleep time. Increasing sleep time to a very high value will lead to poor response time of the
CapSense button. As a result, the sleep time must be based on system requirements.
In any application, if both power consumption and response time are important parameters to be considered, then
you can use an optimized method incorporates both continuous-scan and sleep-scan modes. In this method, the
device spends most of its time in sleep-scan mode, where it scans the sensors and goes to sleep periodically, as
explained in the previous section, thereby consuming less power. When a user touches a sensor to operate the
system, the device jumps to continuous-scan mode, where the sensors are scanned continuously without invoking
sleep, giving very good response time. The device remains in continuous-scan mode for a specified timeout period. If
the user does not operate any sensor within this timeout period, the device jumps back to the sleep-scan mode.
5.5.4 Measuring Average Power Consumption
The following instructions describe how to determine average power consumption when using the sleep-scan
method:
1. Build a project that scans all of the sensors without going to sleep (continuous-scan mode). Include a pin-toggle
feature in the code before scanning the sensors. Toggling the state of the output pin serves as a time marker that
can be tracked with a scope.
2. Download the project to the CapSense device and measure the current consumption. Assign the measured
current to I
ACT
.
3. Get the sleep current information from the datasheet and assign it to I
Slp
.
4. Monitor the toggling output pin with an oscilloscope and measure the time period between two toggles. This
gives the active time. Assign this value to t
Act
.
5. Apply sleep-scan to the project. The time period of the sleep-scan cycle, T, is set by selecting the sleep timer
frequency in the global resources window, as shown in Figure 5-1.
6. Subtract active time from the sleep-scan cycle time period to get the sleep time. t
Slp
= T - t
Act
.
7. Calculate the average current using Equation 11.
8. Calculate average power consumption using Equation 12.
Figure 5-1. Global Resources Window