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From: Pancho <Pancho.Jones@proton.me>
Newsgroups: comp.sys.raspberry-pi
Subject: Re: Need help with PI PICO...
Date: Fri, 29 Mar 2024 13:14:08 +0000
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On 29/03/2024 10:49, The Natural Philosopher wrote:
> On 29/03/2024 00:52, Pancho wrote:
>> On 27/03/2024 10:13, The Natural Philosopher wrote:
>>> On 26/03/2024 21:58, Pancho wrote:
>>>> On 25/03/2024 15:57, The Natural Philosopher wrote:
>>>>> On 25/03/2024 11:31, Pancho wrote:
>>>>>> On 24/03/2024 11:13, The Natural Philosopher wrote:
>>>>>>
>>>>>>> However on trawling the internet I discovered a conversation  
>>>>>>> with someone else *on here* (c.s.r-pi) last year, who was finding 
>>>>>>> that *sleep_us(x)* was highly inconsistent for certain (small) 
>>>>>>> values of x. Sometimes taking a day to end.
>>>>>>>
>>>>>>> Further investigation reveals that in fact it really is a 'sleep' 
>>>>>>> with the processor being put in low power mode and requiring an 
>>>>>>> interrupt to 'wake it up'.
>>>>>>>
>>>>>>
>>>>>> Why not use threads/timers, wait on a lock/semaphore rather than 
>>>>>> sleep.
>>>>>>
>>>>> Good point Pancho, but I was really looking for the simplest code 
>>>>> possible.  Interrupts can be tricky things on a platform whose 
>>>>> architecture you do not understand completely. In any case it was a 
>>>>> learnning curve I preferred not to negotiate if i didnt need to
>>>>>
>>>>
>>>> A timer isn't complicated, just a call back routine, and a 
>>>> semaphore. Interrupts are something an OS does, not me :o). I hate 
>>>> multithread code, but async handling of an external resource is one 
>>>> of the two main places I would use another thread.
>>>>
>>>> I had a look at your code, it looks extraordinarily like a python 
>>>> example on Tom's hardware.
>>>>
>>> Oh. The manufacturers sample code is the source of ALL the 'examples' 
>>> that other people publish as their own., I am just being honest :-)
>>>
>>>> I'm not clear how many times it is succeeding vs failing, but I 
>>>> suspect you really need to bite the bullet and introduce 
>>>> timeouts/error handling, if it fails try again, average out multiple 
>>>> results. i.e. accept it as flawed and that results are statistical, 
>>>> like GPS.
>>>>
>>> Well the averaging out will happen anyway at the server side. I make 
>>> the clients as simple as possible for resilience. In practice oil 
>>> levels only change quickly if you have had the oil stolen overnight 
>>> or if a supplier has filled the tank up, so gross deviations can have 
>>> code exceptions, the rest would be the running average of maybe 20 
>>> samples.
>>> BUT it isn't inaccuracy that worries me per se, it's that it may be 
>>> an indication of underlying timing issues.
>>>
>>>> In many ways the resilience code will be simple, because it is just 
>>>> normal code, rather than cargo culting a novel ultrasonic device.
>>>>
>>> In fact the code in either case is simple.
>>>
>>
>>
>> I understood the idea of a ping delay time. It is just my experience 
>> that things rarely work exactly as I expect them to.
>>
>> FWIW, I'd also massively underestimated the difficulty of coding the 
>> PICO, I'd assumed it was running a multitasking OS, like busybox, but 
>> I see it isn't. I guess there are a whole bunch of gotchas there too.
>>
>>> It is:  send a 10µs pulse to the unit, wait for the echo pulse start 
>>> , get the time, wait for the echo pulse end, get the time, subtract 
>>> the difference.
>>>
>>
>> I'm unclear on terms, but that sounds like the length of the pulse, 
>> 10µs. Not the distance travelled by the pulse. Surely, you should be 
>> measuring the time between sending the pulse and receiving the pulse. 
>> I've probably misunderstood something, if the code is giving a 
>> sensible distance.
>>
> No.
> Maybe ascii art will help
> 
> CONTROL=10µs
> ____| |________
> 
> RETURN = wahatever
> _____|^^^^^^^^^^^^|____________
> 

Yeah, I know what a ping is supposed to do. It is the time interval 
between sending a ping and receiving the echo, simples. But... that 
isn't what your code looks like.

You have:

  while(!gpio_get(ULTRASONIC_IN));		
     //read clock and store
     start=get_absolute_time ();

Which is presumably waiting for silence, no echo, it might work if that 
is the default start state, i.e. if it does nothing, but it is 
redundant. You should start the time interval when the ping is sent.



>>
>>> What appears to be happening is that at short range the echo pulse 
>>> never starts, or ends before the code is aware of it.
>>>
>>>> You can investigate further, by recording fail stats, as well as 
>>>> distance stats.
>>>>
>>> Failure is very very rare. I am sampling for test purposes once a 
>>> second, and its usually an hour or more before it locks up.
>>>
>>> I could simply  turn the while loop into a for loop with a counter so 
>>> that even if I got a null result it wouldn't lock up. Missing one 
>>> sample is no big deal: just take another!
>>>
>>> I am slightly curious as to how  the PICO could miss what is a 
>>> several hundred microsecond wide pulse.
>>>
>>
>>
>> Maybe ultrasound is everywhere. Maybe a bird sings, or a walwart noise 
>> interferes. The device may just move in mysterious ways.
>>
> 
> No, I'ts definitely all associated with a short return pulse
> 

A short pulse always fails? I wasn't clear if it just made it more 
vulnerable to failure. Failure caused by something else? Maybe 
wavelength? Interference. I really don't know. I don't know how you can 
rule stuff out. Apart from empirically, test reliability, in which case 
you need to record failure stats as well as success.

>>> So far I have managed to get stuff reliable without having to unpick 
>>> the ARM interrupt pandora's box. I am keen to leave it closed. The 
>>> LWIP stack was bad enough...:-)
>>>
>>
>> Yeah, I went off the idea of getting a PICO the moment I realised it 
>> didn't have a proper OS. I have spare rPi3s I could use, and I'm 
>> willing to accept high power usage of a couple of watts.
>>
> Well this has to be battery powered.
> 
> You can get some sort of Free BSD RTOS port to a Pico, but in fact 
> mostly what you tend to be doing is just  one thing at a time and so 
> linear code with callbacks works pretty well
> 

Yeah, you say that, but virtually all my experience is based upon a 
multitasking OS: VMS, Unix, Windows NT, Linux. I can't remember stuff I 
did in the early 1980s. I have a huge store of experience, intuition, in 
the multitasking Posix like world, none in single process.

I'm a little paranoid, pessimistic, I think the world is out to mess me 
up. If things can go wrong, they will go wrong, which is why I'm 
unwilling to step into such a different paradigm.