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AN1775 Datasheet(PDF) 8 Page - Freescale Semiconductor, Inc |
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AN1775 Datasheet(HTML) 8 Page - Freescale Semiconductor, Inc |
8 / 12 page Application Note AN1775 Rev. 1 The only exception is key 1, which connects VSS to the ADC input and 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 |
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