ADC conversion time varies unexpectedly for single channel

Hello all,

I am using PTP example to synchronise time and after that starting timer with pulse time (positive width) 10 us and period 20 us.
I am using IPLS and MINT interrupts to detect rising and falling edge, so that I have either interrupt at 10 us interval.
In both timer edge detection  ISR I am using software ELC event to trigger ADC on single channel. So Basically ADC is triggered at every 10 us on single channel.
I am using ADC 12 bit conversion in normal mode without sample and hold activation on ch 0 in ADC unit 0 on PIN P000.
The ADC ISR is triggered after AD conversion is finished. ADC conversion end interrupt has highest priority (0). In the ADC ISR GPIO PIN is raised high, read ADC result in a variable and GPIO is lowered.

The problem is ADC conversion time varies after trigger. Like most of the time it takes around 5us but some times it takes more than 10 us as well. And this is causing troubles as next trigger is missed.

Can any one suggests how to make the conversion time deterministic?

See the following screen grab. The top window is the 10 us triggers. At all the edges I am triggering ADC with software ELC link. The next window immediately below the first window shows 'persistence' and the trigger is quite deterministic meaning not varying in time. The third window shows GPIO toggle in ADC ISR. Which generally is seen after 5 us on an average after each edge in the upper window, The interesting observation is the bottom most window which is the variance of ADC scan finish ISR. This seems to vary a lot. You can see the negative pulse width measurement varies from 2.9899 us to 20.039us which is huge undesired range. Given ADC single scan is triggered at every 10 us, the gap between this should be 10 us as can be seen in mean value which is 9.7067 us, but wild variation in minimum and maximum values are quite derailing the operation of signal acquisition.

Any lead in making the ADC conversion time more deterministic will be highly appreciated.


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  • Hi AZ,

    I think the issue is interrupt latency of IPLS and MINT interrupts. I am toggling GPIO just before software ELC events and I see the results are quite startling. I have put highest priority to both the interrupts and still ISR is called after non deterministic time. I have event linked to GPIO which toggles the pin and that is quite accurate but till the time ISR is called it is very random. see the same picture but now in the bottom part is the glitch when I trigger ADC. And you see ADC is itself triggered at varying time after interrupts.

    So my question is now how to minimise the interrupt latency in the case of IPLS and MINT interrupts?

  • So, the issue is not the conversion time of ADC but the different time intervals in order to trigger the ADC, right ?

    Are you using the Event Link Controller with EPTPC interrupts as source signals and ADC trigger as output signals to start the conversion?

    In this case there is no CPU intervention so the interrupt latency should not be an issue.

  • I am using EPTPC timer interrupts to trigger ADC. I am generating software events using function 'R_ELC_SoftwareEventGenerate' in both ISRs. Pin toggling is done by hardware events.

    I can not use two separate events to trigger single ADC that is why I am using software events.

    one is triggered on rising edge from MINT ISR and other is triggered falling edge IPLS ISR.

    The thing is 10us latency in servicing these ISR even at highest priority is simply unacceptable. 

    Can you please suggest how to reduce these latency ?

  • Do you have other threads in your application apart from the PTP thread ?

  • I am using/modifying  the PTP example and I haven't added any more threads than the example uses.

    Also I am trying to use only IPLS interrupt on both rising and falling edges to trigger hardware ELC events , but it seems ADC is not getting triggered.

    Also on page#900 there is no mention of IPLS trigger but in code it is possible to link. Can you clarify if IPLS can be linked with ELC or not ?

  • Yes, the ELC_EVENT_EPTPC_IPLS event can be linked with ELC. 

    Can you make sure that the IPLS interrupts occur ?

    Do you call R_ELC_Open and then R_ELC_Enable in your code ?

    The example is using a RTT Thread. Can you try to delete this ?

    Please let us know.

  • Yes, the ELC_EVENT_EPTPC_IPLS event can be linked with ELC. 

    I tried but it is not working . I am having IPLS ISR from where I am changing GPIO level using 'R_IOPORT_PinEventOutputWrite'

    Even in hardware manual there is no mention of this event in ELC chapter as well as in EPTPC chapter.