|BITSCOPE||design||manual||modes||Slow Clock/Time-base Expansion (R13)|
Bitscope normally acquires data at 25 MS/s or 40 MS/s
However, sometimes such a high capture rate is not what you want.
Trace modes TM2 and TM3 use time-base expansion and trace mode TM4 slow clock operation to allow the effective sample rate to be reduced for those applications that may require a low sample rate to capture signals of lower bandwidth data over a longer period of time.
The time-base expansion trace modes 2 and 3 use the Time-base Expansion Register R13 to slow data capture by inserting a repeating pause state during data acquisition. The captured data therefore consists of a series of small sample data sets which are actually sampled at full speed (ie, the A/D convertor still operates at 25 or 40 MS/s).
The average of the sample sets defines the sample value at the expanded (ie, slower) rate.
The advantage of time-base expanded data capture is that high frequency statistics (such as short term jitter or noise) can be determined from the same sample set.
If you are not interested in the high frequency statistics, you can use the Slow clock trace mode as an alternative to time-base expansion. Slow clock mode uses the same register R13 to actually slow the sample clock used by the A/D convertor and as such requires that the convertor you're using can be clocked at the lower rate.
In the case of the Motorola MC10319 the sample clock may be driven to very low sample rates. The lowest sample rate supported in slow clock mode is 3 kHz which allows the continuous capture of up to 5 seconds of data.