External ADC trigger from a high clock (via Timer)

PAk

Hello.

One of our clients needs to provide to our Tiva board an external reference clock for sampling some signal. In specific, they will provide a 30MHz signal, and they want the Tiva board to latch that signal to sample (at a lower rate,around 300khz) with the ADCs (of course using uDMA). Yes, coherent system.

We think that using a GPIO as ADC_TRIGGER_EXTERNAL is not valid, because the memory depth won't be enough to hold that data and use some oversampled signal.

Our idea goes through taking that signal to a Timer pin, use a counter to lower that clock and select ADC_TRIGGER_TIMER as parameter, probably via TIMER_CFG_A_CAP_COUNT.

Would it be possible to do that?

Thank you

over 9 years ago

0 Amit Ashara over 9 years ago

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Hello PAk,

Is this for a TM4C123x or a TM4C129x device?

0 PAk over 9 years ago in reply to Amit Ashara

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Amit Ashara said:
Hello PAk,
Is this for a TM4C123x or a TM4C129x device?

129x , sorry.

0 Amit Ashara over 9 years ago in reply to PAk

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Hello PAk,

There is an issue here. The Timer Enable is auto cleared when the match occurs. So there would be few edges that would be missed while the timer gets re-enabled.

0 PAk over 9 years ago in reply to Amit Ashara

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I see.

Is there any way to auto reload the timer? Actually, it will always be on.

On the other hand, would it be possible to lower that signal and sending it to any other timer or pwm? The idea is to use one of every x Edges for converting, periodically.

0 Amit Ashara over 9 years ago in reply to PAk

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Hello PAk

Unfortunately no other timer module like PWM can perform the task. Auto reload can be done using DMA but it cannot guarantee that all edges will be captured. It can be 1-3 missing edges.

0 PAk over 9 years ago in reply to Amit Ashara

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Amit Ashara said:
Hello PAk

Unfortunately no other timer module like PWM can perform the task. Auto reload can be done using DMA but it cannot guarantee that all edges will be captured. It can be 1-3 missing edges.

Hello Amit, looking for another approximation I found this thread:

In it, your workmate Chuck Davenport says:

"After the match value is reached in down-count mode, the counter is then reloaded using the value
in GPTMTnILR and GPTMTnPR registers, and stopped because the GPTM automatically clears
the TnEN bit in the GPTMCTL register. Once the event count has been reached, all further events
are ignored until TnEN is re-enabled by software. In up-count mode, the timer is reloaded with 0x0
and continues counting."

So maybe sending the 30MHz to a Timer and setting it up as TIMER_CFG_A_CAP_COUNT_UP, could work beacuse in UP mode it autoreloads.

Could this be a solution? If not, why?

0 Amit Ashara over 9 years ago in reply to PAk

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Hello PAk,

CPU is working at 120MHz. And the Timer input is at 30MHz. So the CPU has maximum of 4 clocks after the interrupt edge to re-enable the timer. The interrupt generation will cost 1 clock and then Interrupt Handler invocation would take 10-12 clocks. Furthermore the code execution overhead. The maths is evident.

0 PAk over 9 years ago in reply to Amit Ashara

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Ok.....and is there any way to rescale (lower freq.) an external signal to another Timer/pin?
I thought PWM could do this.

0 Robert Adsett over 9 years ago in reply to PAk

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I expect this is what you are trying to avoid but could you just use an external divider?

Robert

0 PAk over 9 years ago in reply to Robert Adsett

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I wouldn't like to change the HW, so I would like to try all possibilities.
Thank you Robert

0 Amit Ashara over 9 years ago in reply to PAk

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Hello Pak

The RTC mode can be flexed to do something similar, but the first trigger may be a problem.

0 PAk over 9 years ago in reply to Amit Ashara

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Amit Ashara said:
Hello Pak

The RTC mode can be flexed to do something similar, but the first trigger may be a problem.

Could you explain a little bit? Seems interesting

0 Amit Ashara over 9 years ago in reply to PAk

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Hello PAk,

The first time it is enabled it counts from 1 but on subsequent reloads it counts from TAILR. I would ask you to read the datasheet section so that you can be better informed on the questions you need to ask and tests that you may need to formulate.

0 PAk over 9 years ago in reply to Amit Ashara

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I was thinking about using an SPI port as slave (30MHz as CLK), and use the CS signal as divided clock.

0 Amit Ashara over 9 years ago in reply to PAk

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Hello PAk

The SPI clock in slave mode is limited to 1/12th of the system clock and that would be 10MHz for a system clock of 120MHz. This will not work.

0 PAk over 9 years ago in reply to Amit Ashara

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Amit Ashara said:
Hello Pak

The RTC mode can be flexed to do something similar, but the first trigger may be a problem.

I read and checked but didn't find any example with RTC and what I try to achieve.

On the other hand,would it be possible to generate a Timer (or PWM) signal, synchronized with the external 30MHz. I wouldn't mind to take it again in for ADC triggering on another pin.

Regards

0 Amit Ashara over 9 years ago in reply to PAk

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Hello PAk,

In which case I would suggest that Robert's earlier suggestion of dividing down the clock and then using GPIO trigger would be very optimal.

0 PAk over 9 years ago in reply to Amit Ashara

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Amit Ashara said:
Hello PAk,

In which case I would suggest that Robert's earlier suggestion of dividing down the clock and then using GPIO trigger would be very optimal.

Ok, but I would like to divide down the trigger with the MCU. Wouldn't it be possible with timers or PWM as inputs?

For example, setting a timer to count up (autoreload), without interruptions, that generates an edge every time that the TimerMatchSet value is reached.

0 Amit Ashara over 9 years ago in reply to PAk

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Hello PAk

As I mentioned not at 30MHz. I would suggest evaluating the RTC mode by writing a sample code.

0 PAk over 9 years ago in reply to Amit Ashara

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Amit Ashara said:
As I mentioned not at 30MHz. I would suggest evaluating the RTC mode by writing a sample code.

Could you share some example?

0 Amit Ashara over 9 years ago in reply to PAk

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Hello PAk,

I do not have such a code available with me.

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External ADC trigger from a high clock (via Timer)