How to freeze the trace to see the whole packet ?

[atferrari]

How to freeze the trace?

I've finally got a 16F877 generating a command, as per the SIRC protocol (Sony IR command) with (1- start + 7- command + 5- device) pulses and just for testing, the signal is emitted through a continuous loop.

I've spent quite some time trying to freeze the trace in my scope to actually count the pulses and measure periods but to no avail. I can frezze two or three pulses but if I see the whole "packet" I can't count / measure anything.

My scope, far from a DSO is a imple analogic 10 MHz dual channel. What's the simplest setup to be able to get a steady image on the screen? Any trick I could play to be able to explore the packet along the full length :?:

While typing this I realized that a certain instability is due to a TMR0 interrupt providing a time base for other purposes. I forgot to disable it and will be the first thing to do tomorrow at the bench.

Suggestions / comments appreciated :!:

 

[echo47]

How to freeze the trace?

Does your scope have delayed sweep? That would let you zoom in and slowly walk along the packet. Hopefully your packet is repetitive, because an analog scope can't store anything.

I suppose most 10 MHz scopes don't have delayed sweep. This may be a good time to go shopping for a fancier scope. A digital scope would be even better, because it can store the entire packet and let you zoom in and examine it later.

In the mean time, you could build your own simple delayed sweep circuit. It's just an adjustable timer. The start of your packet somehow starts the timer. When the timer runs out, it triggers your scope. Set your scope to a fast sweep rate so you can see individual pulses. Vary the timer to walk along the packet.

 

[VVV]

Re: How to freeze the trace?

You can try what echo47 is suggesting, build a timer to delay the triggering of the time base. You will have to use the EXT. trigger, of course, to let the timer trigger the scope, not the signal. But you need a way to trigger the timer. For digital signals, it should be very easy.

Or, if your scope has a x10 magnifier, turn it on to expand the waveform and then adjust the x-pos to 'scroll'.

 

[atferrari]

Done!

A quite simple solution with good results: a pin going high for three instruction cycles in Chanel 1 (before the packet), got a stable image (in Chanel 2) as needed.

Thanks for replying.