Re: [beagleboard] Phasors calculation using libpruio

[John Syne]

> On Feb 2, 2017, at 11:43 AM, Renzo Fabián  wrote:
> 
> Thank you TJF and John for share your knowledge.
> 
> Actually, I have in mind two projects based on phasor calculation:
> 1. A low-cost PMU (Phasor measurement unit) that requires the phasors 
> calculation and then send to a collector.
You need to consider the time critical nature of communicating the phasor 
measurements to the collector. How familiar are you with Synchrophasor systems? 
Are you planning on updating 30 or 60 times per second? You will also need 
time/frequency synchronization. Any idea how you plan to do this?

> 
> 2. A low-cost protection relay that requieres the phasors calculations, 
> operations between these phasors and responses (digital outputs) every each 
> fixed time. 960 Sps por each analog input and 280 Sps for each digital output.

Here again, this is a time critical operation and you need to respond within 
20mS. In this case, I would agree with TJF, the PRU would be the better 
approach. 

Regards,
John 
> 
> I have began code and then I hope share it with you.
> 
> Regards.
> 
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Re: [beagleboard] Phasors calculation using libpruio

[TJF]

Am Dienstag, 31. Januar 2017 19:03:04 UTC+1 schrieb john3909:
>
> It is only the period between the samples that is important to be 
> deterministic. After that, it doesn’t matter if there is any latency unless 
> he is using the calculations in a control loop. If he is just doing phasor 
> measurements, then latency isn’t important. You need to understand the 
> application before you make assumptions like this. 
>
 
Ahhh, now I understand!

We should use two different solutions for measurement and closed loop 
control ...

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Re: [beagleboard] Phasors calculation using libpruio

[John Syne]

> On Jan 31, 2017, at 7:19 AM, TJF  wrote:
> 
> 
> 
> Am Montag, 30. Januar 2017 18:36:37 UTC+1 schrieb john3909:
> Well, again it depends on what you mean by realtime. To most developers, it 
> means deterministic
> 
> Yes, deterministic:
> steady latency
> the lowest possible steady latency
> and since the AM335x ADC uses a sequencer to do the sampling, it is 
> deterministic,
> 
> The sequencer garanties a deterministic process from analog line to the FiFo 
> buffer. You know that you cannot compute inside the FiFo. The data has to get 
> to a processor register first. And the complete process, from analog line up 
> to the processor register, has to be deterministic.
>  
> and hence there is no difference between the PRU and ARM with DMA.
> 
> There is a big difference, since the ARM gets loaded by other kernel tasks. 
> On Beaglebone hardware under LINUX, the PRU is the only system that can 
> garanty deterministic sampling (when bus access is unlimited).
It is only the period between the samples that is important to be 
deterministic. After that, it doesn’t matter if there is any latency unless he 
is using the calculations in a control loop. If he is just doing phasor 
measurements, then latency isn’t important. You need to understand the 
application before you make assumptions like this. 

Regards,
John
>  
> This shouldn’t be a pissing match as to how fast you can sample, but rather 
> focus on what the OP application was. In this case, since the PRU doesn’t 
> have the required math functions,
> 
> The math in the original post contains simple table lookups, additions and 
> multiplications. It's easy to implement that on the PRU (, but not necessary 
> for 6 kSps).
>  
> the PRU is only doing what the DMA is doing, moving samples from the FIFO to 
> DDR memory. Hence, why waste the PRU on such a trivial function. Rather save 
> it for something more useful.
> 
> Yes, don't waste the PRUSS! That's why I didn't propose any customized 
> firmware, but libpruio. Even if Fabián doesn't need the digital features 
> (GPIO, PWM, CAP, QEP), he might use the trigger features to start the 
> measurement at the same phasing, in order to make results better comparable.
> 
> Regards
> 
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Re: [beagleboard] Phasors calculation using libpruio

[TJF]

Am Montag, 30. Januar 2017 18:36:37 UTC+1 schrieb john3909:
>
> Well, again it depends on what you mean by realtime. To most developers, 
> it means deterministic 
>

Yes, deterministic:

   1. steady latency
   2. the lowest possible steady latency

and since the AM335x ADC uses a sequencer to do the sampling, it is 
> deterministic,
>

The sequencer garanties a deterministic process from analog line to the 
FiFo buffer. You know that you cannot compute inside the FiFo. The data has 
to get to a processor register first. And the complete process, from analog 
line up to the processor register, has to be deterministic.
 

> and hence there is no difference between the PRU and ARM with DMA.
>

There is a big difference, since the ARM gets loaded by other kernel tasks. 
On Beaglebone hardware under LINUX, the PRU is the only system that can 
garanty deterministic sampling (when bus access is unlimited).
 

> This shouldn’t be a pissing match as to how fast you can sample, but 
> rather focus on what the OP application was. In this case, since the PRU 
> doesn’t have the required math functions, 
>

The math in the original post contains simple table lookups, additions and 
multiplications. It's easy to implement that on the PRU (, but not 
necessary for 6 kSps).
 

> the PRU is only doing what the DMA is doing, moving samples from the FIFO 
> to DDR memory. Hence, why waste the PRU on such a trivial function. Rather 
> save it for something more useful.
>

Yes, don't waste the PRUSS! That's why I didn't propose any customized 
firmware, but libpruio. Even if Fabián doesn't need the digital features 
(GPIO, PWM, CAP, QEP), he might use the trigger features to start the 
measurement at the same phasing, in order to make results better comparable.

Regards

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Re: [beagleboard] Phasors calculation using libpruio

[John Syne]

Well, again it depends on what you mean by realtime. To most developers, it 
means deterministic and since the AM335x ADC uses a sequencer to do the 
sampling, it is deterministic, and hence there is no difference between the PRU 
and ARM with DMA. This shouldn’t be a pissing match as to how fast you can 
sample, but rather focus on what the OP application was. In this case, since 
the PRU doesn’t have the required math functions, the PRU is only doing what 
the DMA is doing, moving samples from the FIFO to DDR memory. Hence, why waste 
the PRU on such a trivial function. Rather save it for something more useful. 

One more thing, the IIO framework is capable of GSPS performance. 

Regards,
John




> On Jan 30, 2017, at 9:17 AM, TJF  wrote:
> 
> 
> 
> Am Sonntag, 29. Januar 2017 21:50:31 UTC+1 schrieb john3909:
> Hi TJF,
> 
> The IIO ADC can sample at 800K samples per second [1].
> 
> This is not real-time sampling! When you transfer chunks of samples by DMA, 
> you add further and unsteady latency. The only way to sample real-time is 
> pulling samples one by one from the FiFo, and this is limited to 200 kSps (= 
> TI specification, on some boards I got up to 250 kSps).
>  
> You cannot achieve that sampling rate with with PRU and have it do something 
> useful with those samples.
> 
> Why do you know what I can achieve? I did experimental sampling at 1600 kSps 
> with the PRU, up to 295 samples (until the FiFo overflows). I assume that I 
> can get twice that number by using both FiFos (with more than one channel). 
> But this is not real-time, and it's not ready for daily use.
> 
> BTW: At 200 kSps the PRU performes 1000 instructions between two samples. 
> Fairly enough to do something useful (as long as you don't use remoteproc 
> framework).  
> 
> Please do not confuse the users! You should mention that your solution is 
> experimental and you're looking for beta testers.
> 
> I don’t see the purpose of having PRU read from /dev/iio:deviceX.
> 
> Not today, but in future you may notice that the PRUSS are very good in data 
> processing and you'll need to feed them with data for that purpose.
> 
> 
> @Fabián
> 
> The external ADC is an option when you may need the TSC_ADC_SS for a touch 
> screen on the board. Otherwise it's an overkill, just protect the analog 
> input lines against under-/overvoltage.
> 
> Regards
> 
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Re: [beagleboard] Phasors calculation using libpruio

[TJF]

Am Sonntag, 29. Januar 2017 21:50:31 UTC+1 schrieb john3909:
>
> Hi TJF,
>
> The IIO ADC can sample at 800K samples per second [1]. 
>

This is not real-time sampling! When you transfer chunks of samples by DMA, 
you add further and unsteady latency. The only way to sample real-time is 
pulling samples one by one from the FiFo, and this is limited to 200 kSps 
(= TI specification, on some boards I got up to 250 kSps).
 

> You cannot achieve that sampling rate with with PRU and have it do 
> something useful with those samples.
>

Why do you know what I can achieve? I did experimental sampling at 1600 
kSps with the PRU, up to 295 samples (until the FiFo overflows). I assume 
that I can get twice that number by using both FiFos (with more than one 
channel). But this is not real-time, and it's not ready for daily use.

BTW: At 200 kSps the PRU performes 1000 instructions between two samples. 
Fairly enough to do something useful (as long as you don't use remoteproc 
framework).  

Please do not confuse the users! You should mention that your solution is 
experimental and you're looking for beta testers.

I don’t see the purpose of having PRU read from /dev/iio:deviceX.
>

Not today, but in future you may notice that the PRUSS are very good in 
data processing and you'll need to feed them with data for that purpose.


@Fabián

The external ADC is an option when you may need the TSC_ADC_SS for a touch 
screen on the board. Otherwise it's an overkill, just protect the analog 
input lines against under-/overvoltage.

Regards

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Re: [beagleboard] Phasors calculation using libpruio

[John Syne]

> On Jan 29, 2017, at 2:17 PM, Renzo Fabián  wrote:
> 
> I understand there are many ways to solve the problem.
> 
> 1. Using PRUs via libpruio suggested by TJF.
> 2. Using DMA via iio driver suggested by John.
Search for iio_generic_buffer, which is in mainline under tools/iio. This will 
show you how to read the IIO buffer from a user space app. 

Alternatively, you can use iiod which you will find in libiio, which allows you 
to read the samples remotely. You can use IIO_Oscilloscope to connect to iiod. 

https://wiki.analog.com/resources/tools-software/linux-software/iio_oscilloscope
 


Regards,
John

> 3. Using a external ADC suggested by Woody.
> 
> I understand the solution 1.
> 
> John, can you send a example of ADC sampling and DMA?
> 
> For 3, I don't want to use a external ADC for this project. But, for others 
> I'll have in mind this solution. This solution would be possible with RPi3?
> 
> Thanks you.
> 
> 
> El viernes, 27 de enero de 2017, 2:10:27 (UTC-2), Renzo Fabián escribió:
> 
> I have the following C code's parts that uses tables of sins and cosins to 
> calculate a fundamental phasor of 60 Hz electrical system using 16 samples 
> per cycle (960 Sps):
> 
> float cosins[]={1, 0.92388, 0.70711, 0.38268, 0, -0.38268, -0.70711, 
> -0.92388, -1,
>-0.92388, -0.70711, -0.38268, 0, 0.38268, 0.70711, 0.92388};
>   
> float sins[]={0, 0.38268, 0.70711, 0.92388, 1, 0.92388, 0.70711, 0.38268, 0,
>  -0.38268, -0.70711, -0.92388, -1, -0.92388, -0.70711, 
> -0.38268};
> 
> ...
> 
> for(i=0; i <16; i++){
>ret_r=x[15-i]*cosins[i]+ret_r;
>ret_i=x[15-i]*sins[i]+ret_i;
>}
> 
> return 0.088388*ret_r -0.088388*ret_i*I;
> 
> Is it possible to perform this using the libpruio in order to read 6 analog 
> inputs? If yes? What is the best way to implement it if other operations were 
> need, i.e. compute the division of 2 phasors.
> 
> Thanks.
> 
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Re: [beagleboard] Phasors calculation using libpruio

[John Syne]

Hi TJF,

The IIO ADC can sample at 800K samples per second [1]. You cannot achieve that 
sampling rate with with PRU and have it do something useful with those samples. 
 

[1] https://patchwork.kernel.org/patch/9391499/ 


I don’t see the purpose of having PRU read from /dev/iio:deviceX. This is for 
ARM only. The OP wanted to do phasor analysis which requires math not easily 
implemented on the PRU. Since the samples are already available on the ARM 
side, I suggested that the OP do phasor analysis on the ARM? 

Regards,
John




> On Jan 28, 2017, at 8:49 PM, TJF  wrote:
> 
> Hi John!
> 
> Am Sonntag, 29. Januar 2017 03:58:21 UTC+1 schrieb john3909:
> The IIO driver is faster and can achieve higher sampling rates.
> 
> The sampling rate limitation is the FiFo buffer in the TSC_ADC_SS. Both 
> drivers can operate at full speed. iio puts more load on the ARM CPU.
>  
> Also the IIO driver is easier to use because you just read from 
> /dev/iio:deviceX.
> 
> Can you please provide some PRU example code how to read from 
> /dev/iio:deviceX? iio is made for ARM side only. In contrast libpruio can 
> provide the data either for the ARM or for the second PRU.
>  
> Regards
> 
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Re: [beagleboard] Phasors calculation using libpruio

[TJF]

Hi John!

Am Sonntag, 29. Januar 2017 03:58:21 UTC+1 schrieb john3909:
>
> The IIO driver is faster and can achieve higher sampling rates.
>

The sampling rate limitation is the FiFo buffer in the TSC_ADC_SS. Both 
drivers can operate at full speed. iio puts more load on the ARM CPU.
 

> Also the IIO driver is easier to use because you just read from 
> /dev/iio:deviceX.
>

Can you please provide some PRU example code how to read from 
/dev/iio:deviceX? iio is made for ARM side only. In contrast libpruio can 
provide the data either for the ARM or for the second PRU.
 
Regards

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Re: [beagleboard] Phasors calculation using libpruio

[John Syne]

> On Jan 27, 2017, at 4:43 PM, Renzo Fabián  wrote:
> 
> john3909,
> 
> there's no reason to use the PRUs. I only need read 6 analog inputs in 
> real-time. I thought the only way would be using the PRUs. And the best way 
> by means of lbpruio.
Well, it all depends on what you mean by realtime? The ADC on the AM3358 uses a 
sequencer that samples the analog channels at a fixed sampling rate and places 
those samples in a FIFO. It does not matter if you use PRU or the IIO driver, 
because the channel sampling occurs at the same time. The difference is the PRU 
samples the ADC status and then copies the data into DDR and the IIO driver 
uses DMA to move the samples into DDR. The IIO driver is faster and can achieve 
higher sampling rates. Also the IIO driver is easier to use because you just 
read from /dev/iio:deviceX. Better still, you have both PRUs available for more 
important work. 

Regards,
John
> 
> El viernes, 27 de enero de 2017, 2:10:27 (UTC-2), Renzo Fabián escribió:
> 
> I have the following C code's parts that uses tables of sins and cosins to 
> calculate a fundamental phasor of 60 Hz electrical system using 16 samples 
> per cycle (960 Sps):
> 
> float cosins[]={1, 0.92388, 0.70711, 0.38268, 0, -0.38268, -0.70711, 
> -0.92388, -1,
>-0.92388, -0.70711, -0.38268, 0, 0.38268, 0.70711, 0.92388};
>   
> float sins[]={0, 0.38268, 0.70711, 0.92388, 1, 0.92388, 0.70711, 0.38268, 0,
>  -0.38268, -0.70711, -0.92388, -1, -0.92388, -0.70711, 
> -0.38268};
> 
> ...
> 
> for(i=0; i <16; i++){
>ret_r=x[15-i]*cosins[i]+ret_r;
>ret_i=x[15-i]*sins[i]+ret_i;
>}
> 
> return 0.088388*ret_r -0.088388*ret_i*I;
> 
> Is it possible to perform this using the libpruio in order to read 6 analog 
> inputs? If yes? What is the best way to implement it if other operations were 
> need, i.e. compute the division of 2 phasors.
> 
> Thanks.
> 
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Re: [beagleboard] Phasors calculation using libpruio

[robertsmaria934 via BeagleBoard]

On Fri, 1/27/17, John Syne  wrote:

 Subject: Re: [beagleboard] Phasors calculation using libpruio
 To: beagleboard@googlegroups.com
 Date: Friday, January 27, 2017, 8:53 PM
 
 Is there a
 reason you don’t do this on the ARM processor? In the v4.8
 kernel, the ADC uses DMA, so you can sample at full
 speed. 
 
 Regards,John
 
 
 
 
 
 
 On Jan 26, 2017, at 8:10 PM, Renzo
 Fabián 
 wrote:
 
 I have the following C code's
 parts that uses tables of sins and cosins to calculate a
 fundamental phasor of 60 Hz electrical system using 16
 samples per cycle (960 Sps):
 float
 cosins[]={1, 0.92388, 0.70711, 0.38268, 0, -0.38268,
 -0.70711, -0.92388, -1, 
              -0.92388, -0.70711, -0.38268, 0,
 0.38268, 0.70711, 0.92388};  float sins[]={0, 0.38268, 0.70711,
 0.92388, 1, 0.92388, 0.70711, 0.38268, 0,                 -0.38268,
 -0.70711, -0.92388, -1, -0.92388, -0.70711,
 -0.38268};
 ...
 for(i=0; i <16; i++){
  
  ret_r=x[15-i]*cosins[i]+ret_r; 
  ret_i=x[15-i]*sins[i]+ret_i;   }
 return 0.088388*ret_r
 -0.088388*ret_i*I;
 Is it possible to perform this using
 the libpruio in order to read 6 analog inputs? If yes? What
 is the best way to implement it if other operations were
 need, i.e. compute the division of 2 phasors.
 
 Thanks.
 
 
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Re: [beagleboard] Phasors calculation using libpruio

[John Syne]

Is there a reason you don’t do this on the ARM processor? In the v4.8 kernel, 
the ADC uses DMA, so you can sample at full speed. 

Regards,
John




> On Jan 26, 2017, at 8:10 PM, Renzo Fabián  wrote:
> 
> 
> I have the following C code's parts that uses tables of sins and cosins to 
> calculate a fundamental phasor of 60 Hz electrical system using 16 samples 
> per cycle (960 Sps):
> 
> float cosins[]={1, 0.92388, 0.70711, 0.38268, 0, -0.38268, -0.70711, 
> -0.92388, -1,
>-0.92388, -0.70711, -0.38268, 0, 0.38268, 0.70711, 0.92388};
>   
> float sins[]={0, 0.38268, 0.70711, 0.92388, 1, 0.92388, 0.70711, 0.38268, 0,
>  -0.38268, -0.70711, -0.92388, -1, -0.92388, -0.70711, 
> -0.38268};
> 
> ...
> 
> for(i=0; i <16; i++){
>ret_r=x[15-i]*cosins[i]+ret_r;
>ret_i=x[15-i]*sins[i]+ret_i;
>}
> 
> return 0.088388*ret_r -0.088388*ret_i*I;
> 
> Is it possible to perform this using the libpruio in order to read 6 analog 
> inputs? If yes? What is the best way to implement it if other operations were 
> need, i.e. compute the division of 2 phasors.
> 
> Thanks.
> 
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Re: [beagleboard] Phasors calculation using libpruio

[Przemek Klosowski]

On Thu, Jan 26, 2017 at 11:10 PM, Renzo Fabián  wrote:
> I have the following C code's parts that uses tables of sins and cosins

PRU uses 32-bit integer arithmetic, and only has unsigned add and
subtract. You'd have to recast your calculation into fixed point and
use software multiplication. What is the required accuracy? If it's
not too much, perhaps you could use table lookup, or combine table
lookup with linear interpolation?

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[beagleboard] Phasors calculation using libpruio

[Renzo Fabián]

I have the following C code's parts that uses tables of sins and cosins to 
calculate a fundamental phasor of 60 Hz electrical system using 16 samples 
per cycle (960 Sps):

float cosins[]={1, 0.92388, 0.70711, 0.38268, 0, -0.38268, -0.70711, 
-0.92388, -1,
   -0.92388, -0.70711, -0.38268, 0, 0.38268, 0.70711, 0.92388};
  
float sins[]={0, 0.38268, 0.70711, 0.92388, 1, 0.92388, 0.70711, 0.38268, 0,
 -0.38268, -0.70711, -0.92388, -1, -0.92388, -0.70711, 
-0.38268};

...

for(i=0; i <16; i++){
   ret_r=x[15-i]*cosins[i]+ret_r;
   ret_i=x[15-i]*sins[i]+ret_i;
   }

return 0.088388*ret_r -0.088388*ret_i*I;

Is it possible to perform this using the libpruio in order to read 6 analog 
inputs? If yes? What is the best way to implement it if other operations 
were need, i.e. compute the division of 2 phasors.

Thanks.

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