tom dunigan / Mbed 2 deprecated k64f_rtcisr

measure RTC crystal freqeuncy against MCU crystal

Dependencies:   mbed

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This program uses the RTC 1-second interrupt to capture the Timer microsecond value to estimate the error in the RTC's 32KHz crystal. (the K64F MCU's crystal could be in error as well, though mine was within 20 ppm). This program measured the error at about 250 ppm, well out of the 20ppm spec for the RTC crystal???

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