Chapter 9 Counters

80 MHz Timebase. Your measurement may return 1600 ± 1 cycles depending on the phase of the signal with respect to the timebase. As your frequency becomes larger, this error of ±1 cycle becomes more significant; Table 9-2illustrates this point.

Table 9-2.Frequency Measurement Method 1

Task

Equation

Example 1

Example 2

 

 

 

 

 

 

 

 

Actual Frequency to Measure

F1

50 kHz

5 MHz

 

 

 

 

Timebase Frequency

Ft

80 MHz

80 MHz

 

 

 

 

Actual Number of Timebase

Ft/F1

1600

16

Periods

 

 

 

 

 

 

 

Worst Case Measured Number of

(Ft/F1) – 1

1599

15

Timebase Periods

 

 

 

 

 

 

 

Measured Frequency

Ft F1/(Ft – F1)

50.125 kHz

5.33 MHz

 

 

 

 

Error

[Ft F1/(Ft – F1)] – F1

125 kHz

333 kHz

 

 

 

 

Error %

[Ft/(Ft – F1)] – 1

0.06%

6.67%

 

 

 

 

Method 1b (measuring K periods of F1) improves the accuracy of the measurement. A disadvantage of Method 1b is that you have to take K + 1 measurements. These measurements take more time and consume some of the available USB bandwidth.

Method 2 is accurate for high frequency signals. However, the accuracy decreases as the frequency of the signal to measure decreases. At very low frequencies, Method 2 may be too inaccurate for your application. Another disadvantage of Method 2 is that it requires two counters (if you cannot provide an external signal of known width). An advantage of Method 2 is that the measurement completes in a known amount of time.

Method 3 measures high and low frequency signals accurately. However, it requires two counters.