Application Note

AN1775 Rev. 1

needs no resistor.

By assigning a range of ADC conversion values to each key, the user

can provide a fair amount of error padding. Considerations include

resistor tolerance, ADC accuracy, and parasitic time constants. The

range of conversion values for a particular keypress can be narrowed to

improve the accuracy of the application. Or, for “quick and dirty”

arbitration, keep the range as wide as possible.

Using a Single

Resistor Value

The method presented here used different resistor values to produce

equal voltage intervals. Another method would be to use the same

resistance value for all segments in the divider. The disadvantage of

using the same resistor values is that it greatly diminishes the effective

range of conversions. Also, if resistances are kept equal, the voltage

step between switches approaches the resolution of the ADC.

Therefore, the error margin diminishes as more keys are added.

Programming

Considerations

There are several ways to implement such an application. One thing to

choose is whether to poll the ADC when desired or link a keypress to an

interrupt source.

Some ADCs continuously convert once enabled, allowing a new value

to be available every 32 clock cycles. Others do a single conversion

when a register is written to and don’t do another conversion until the

register is written to again.

A polling scheme can use a periodic timing source as a signal to poll. For

example, the real-time interrupt (RTI) or timer overflow (TOF) interrupt

can be used to scan the ADC input at a given rate.

Not linking a keypress to an interrupt source can cause timing problems

and might miss a keypress. Careful consideration should be given to

timing and voltage error requirements to determine if this method is

appropriate.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com