AN1250 datasheet(9/22 Pages) MICROCHIP

9 / 22 page

DS01250A-page 9

AN1250

IMPLEMENTATIONS OF SPECIAL

PURPOSE CAPACITIVE TOUCH

This section describes some of the special handling of

the data that is available from reading the capacitive

touch sensors using the CTMU. The following topics

are covered:

• Software algorithms for reliable touch operation

• Software algorithms for special purpose

capacitive touch sensor implementations

• Physical board properties for optimal capacitive

touch sensors

Software Algorithms for Reliable Touch

Operation

There are several software methods used to combat

problems with false sensing of capacitive touch sens-

ing circuits. The following software algorithms are used

not only in the PICDEM Touch Sense 2 Demo Board,

but throughout all of the capacitive touch solutions

offered by Microchip.

• First method – This is to use a slow averaging

routine. This is implemented by simply adding the

value read for any capacitive touch sensor circuit

into a running average. The key to this method is

that a value is only added to the average at a rate

much slower than the actual rate that the

capacitive touch sensor is being read. On the

PICDEM Touch Sense 2 Demo Board, for

example, the average is only updated on every

16th read of the capacitive sensor channel. Since

the update rate is 16 ms, the slower moving

average value is updated at a rate of 256 ms. This

slower moving value is then compared to the

immediate value. If the immediate value varies by

more than what has been established for the trip

point for that capacitive sensor, the sensor is

determined to be “pressed”, or touched. For more

information on the method of slow averaging,

refer to the specific averaging section in AN1103,

“Software Handling for Capacitive Sensing”.

• Second method – This is a debouncing routine.

This algorithm is very similar to that used for

debouncing of mechanical switches. In the case of

capacitive sensors, the debouncing acts as a noise

filtering mechanism. Since noise by definition is

non-periodic, it should not occur repetitively in

multiple sequential scans of a capacitive sensor

channel. Figure 4 provides the routine setup for

both a pressed and unpressed condition. It

requires three consecutive values of pressed or

unpressed before the actual state of the capacitive

touch sensor is ultimately determined. The value

can be changed to suit the application. It should be

noted that if a higher number is used, a slower

response time will be observed.

FIGURE 4:

DEBOUNCE ALGORITHM FLOWCHART

Enter

Read Sensor

Unpressed

Unpressed_Count

= 0

Yes

Pressed

Yes

Exit

Sensor reading

pressed?

Pressed_Count

> = 3?

Pressed_Count ++

Unpressed_Count ++

Pressed_Count

= 0

Unpressed_Count

> = 3?

AN1250 Datasheet(PDF) 9 Page - Microchip Technology

AN1250 Datasheet(HTML) 9 Page - Microchip Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Datasheet Download

Link URL