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The frequency to measured may be observed from four possible inputs:

1. ADC input.
2. Logic data signal bit 7.
3. 1 GHz Pre-scaler input.
4. Any trigger event or signal on any logic input.

Spock option register bits R7:2/1 are programmed to select the input:

Bit	Name	Value	Meaning
2/1	Trig Bit 7 MUX	0	DD7 : Digital Data Bus Bit 7.
		1	Comparator : trigger match comparator signal.
		2	Event 1 : (Pre-scaler output frequency halved).
		3	Event 2 : (ADC input frequency halved).

If the ADC input is selected the Input/Attenuation Register R14 also must be programmed to select the appropriate input and level attenuation for the primary trace channel.

The secondary trace channel is not used.