Digital triggering logic function

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legier
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Joined: Tue Mar 19, 2019 8:18 am

Digital triggering logic function

Post by legier » Tue Mar 19, 2019 8:47 am

Hi,
I wonder if there is any possibility to trogger picoscope with OR function on digital channels? Currently I can only choose logic function between all analog channels and ANDed digital channels. I would like to specify logic function between digital channels.

Gerry
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Re: Digital triggering logic function

Post by Gerry » Thu Mar 21, 2019 4:03 pm

Hi legier,

Unfortunately, there is no direct OR function for digital triggering.

Regards,

Gerry
Gerry
Technical Specialist

Gerry
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Posts: 441
Joined: Mon Aug 11, 2014 11:14 am

Re: Digital triggering logic function

Post by Gerry » Fri Mar 22, 2019 11:38 am

Hi legier,

Actually, looking at this in more detail, what you ask is possible if you have a spare Analogue input Channel, as you could use a combination of logic and digital triggering to perform an OR function, as follows:

If we consider Demorgans theorem from Boolean Algebra where: not([not A] AND [not B] AND [not C]) = not A OR not B OR not C, then using the AND in the digital triggering setup we would just have to invert the inputs and outputs of the digital channels to get your OR function. Inverting the inputs is easy (just choose the opposite level to the one you want) but inverting the outputs is a bit more tricky. But this where the spare input and Logic triggering come in handy, because if you use a rising or falling edge trigger for the Analogue input Channel and tie it to a clock line that should be going high or low for the conditions of your Digital Channels, and then select a Logic Trigger for both the spare Analogue Channel and the Digital Channels, tick the check boxes for both of them to be used, and select the XOR function, then the input of the XOR function should always be high when the logic Triggers are occurring. We can then use the fact that a high on an input of an XOR gate turns it into an inverter for the other input, which solves our problem. So what you have is the following:
Digital OR triggering.png

And I can trigger on X+X+X+X+X+X+X+X+X+X+X+X+X+0+1+0, as (D0 low) OR (D1 high) OR (D2 low), by using the following:
Digital OR triggering Analog tab.png
Digital OR triggering Digital tab.png

(Note that we have to tie the Analogue input Channel to a changing input level as the Trigger will only fire on a changing signal, even when selecting the 'above' or 'below' directions).

Regards,
Gerry
Technical Specialist

Gerry
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Posts: 441
Joined: Mon Aug 11, 2014 11:14 am

Re: Digital triggering logic function

Post by Gerry » Mon Mar 25, 2019 12:13 pm

Hi,

Just for completeness, in case it's not obvious, a multi-input NOR gate Trigger would be the same circuit in the previous post without the XOR (i.e. just the Digital Triggering, no Logic triggering), and a multi-input NAND gate Trigger would be the same circuit in the previous post without inverting the inputs (i.e. using the correct logic levels instead of inverting them).

Regards,

Gerry
Gerry
Technical Specialist

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