How good is piscope?

[jam1]

Hi,

I tried and tried to get some good sampling from piscope but I always get some strange plots. No matter how slow or fast I sample the gpios, some steps are always missing in the clock output.
I tried with SSH from linux.
I tried directly on the Pi with a HDMI monitor and a USB keyboard. Even with no connection to the Ethernet port.

What I am trying to do is a direct connection to an ADC0804 converter.

At this time, I am only testing the clock to at least feed a proper signal to the converter. I cannot get to that stage yet. The maximum speed I want is around 100kHz, which is trivial in this case. Even 100Hz don't give a clean signal for the output pin.

This is on a B+ with a 40 pin cable connection around 30 cm long normally. If I remove the cable, I get the same result.
Can I assumed that the signal is good even if piscope does no show it? I don't want to buy a real analyser just for this.

Is there an image of a good signal trace somewhere that I can compare with and try to replicate? I may have a bad Pi
Thanks

[pluggy]

I would suspect its par for the course running a real time system with very small intervals on a multitasking OS. The OS has stuff to do to maintain itself, and that takes vital clock cycles from the sampling and displaying in the scope software.

[joan]

Try generating activity on an unused gpio.

e.g.

pigs p 4 64

should show a 800Hz PWM signal (64/256) on gpio4.

Change the frequency with

pigs pfs 4 8000

it should then be a 8000Hz signal.

Do they look OK?

[jam1]

Thank you very much joan. I have a perfect clock now.
Now, I do I stop a PWM without stopping the deamon?

[joan]

Thank you very much joan. I have a perfect clock now.
Now, I do I stop a PWM without stopping the deamon?

pigs p 4 0

or

pigs w 4 0

If you can, use a Linux laptop/desktop to display the data. The Pi itself can collect the data at high data rates and send it over a network, but you are expecting too much if you want to display the traces in "real-time" on a Pi.

[FTrevorGowen]

...
If you can, use a Linux laptop/desktop to display the data. The Pi itself can collect the data at high data rates and send it over a network, but you are expecting too much if you want to display the traces in "real-time" on a Pi.

I can confirm that this method works very well indeed (Thank you, @joan). No longer having access to any sort of oscilloscope (the downside of now being a retired physicist), I finally got around to trying out piscope etc. and have managed to display and monitor both I2C and SPI data transfers at a 1Mhz sample rate and thus optmise timing delays in my 'C' demo. software for various LCD displays, operating in "write-only" mode, and partially solve an apparent anomaly between 4-bit and 8-bit interfacing mechanisms (largely due to a bug of mine of course, - although there still remains a puzzle w.r.t. to I2C, which I'm in the process of building a new test circuit for further investigation)
Trev.

[Kai42]

Hi!

Not sure, if I'm in the right forum, but I'll give it a shot. If you feel a different place is more suited to answer my questions, please let me know.

I just started playing around with raspberry pi and switching 433MHz-outlets.
I have brennstuhl-outlets and a 433MHz transmitter and I can switch the outlets wit hthe transmiiter thru codesend no problem, BUT I don't understand the output of piscope while sending codes. Let's start simple:
If I'm not mistaken, a simple
./RPI_utils/codesend 0
should use protocol 1 from RCSwitch.cpp, which in return should send a sync of 1 high pulse followed by 31 low pulses and then my 0 which again is coded as 1 high and 3 lows - all with 350us pulselength ( I know that this is bogus for actual switching any outlets but should keep the matter simple)
Going to the very start of the full sequence, here's what I would expect to see in piscope:
350us high + 31*350us=10.85ms low
followed by my actual '0' i.e. 350us high and 3*350=1050us low.

What I do see when clearing the sample buffer and submitting the codesend 0 is a high of about 720us followed by a low of about 1360us followed by another high of 470us followed yet by a low of about 1240us and so on. - thatis not even close to what I expected.
Am I reading the piscope output wrong or are my expactations wrong / did I misinterpreted the 'protocol 1' - entry of RCSwitch.cpp and what it actually does to the GPIO-pin?

Cheers,
Kai