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參數(shù)資料
| 型號： | AN1775 |
| 廠商： | 飛思卡爾半導(dǎo)體(中國)有限公司 |
| 英文描述： | Expanding Digital Input with an A/D Converter |
| 中文描述： | 擴(kuò)展數(shù)字輸入與A / D轉(zhuǎn)換器 |
| 文件頁數(shù)： | 8/12頁 |
| 文件大小： | 283K |
| 代理商： | AN1775 |
Application Note
AN1775 Rev. 1
The only exception is key 1, which connects V
SS
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.
F
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
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