Asleep and awake

It sleeps between looks

The TTP223 spends almost all its time asleep. In its low-power mode it wakes about every 160 ms, measures the pad, compares, and sleeps again. That is how it idles on about 1.5 µA at 3 V, a figure a coin cell would hardly notice.

The cost is that a touch waits for the next look.

Press Play. The first touch lands while the chip is idling, and SIGNAL stays LOW until the next look finds it. After that the chip is awake and looks about every 20 ms, so the second touch shows almost at once. The positions of the looks are illustrative; the periods are the datasheet's.

The numbers Tontek gives

At 3 V the datasheet gives the response time as at most 220 ms from low-power mode and at most 60 ms once awake. Both are upper limits, not typical times. After the finger lifts, it stays awake for about 12 s, then drops back to sleep.

In the six-pin package on this board the low-power mode is fixed. The larger TTP223 packages have a pin to keep the chip always awake; this one does not.

What it means for a sketch

Nothing, mostly. A finger stays on a pad for a few hundred milliseconds, and both figures are inside that. It matters in two places:

• Timing a touch. A stopwatch started on SIGNAL going HIGH starts late by up to a fifth of a second on the first touch after a rest, and by much less on the next.
• Quick taps. A brush of the pad shorter than one idle gap can fall between two looks and never reach SIGNAL at all.

There is no bounce to handle. A mechanical switch chatters for a few milliseconds as its contacts settle; this chip decides once per look and drives SIGNAL cleanly, so the sketch can act on the first change it sees.