[chbot] PWM challenge

Mon Feb 12 06:24:18 GMT 2024

Thanks

Yes there are a few parts with high resolution timers out there. STM32G4
has an amazing 5.44GHz timer!

For what I needs, I think 144 or 150MHz will do.
To get, say, 25mV output resolution in a buck regulator from 50V requires
1/2000 resolution. With a 144MHz PWM frequency you can get that with a
switching frequency of
144MHz/2000 = 72kHz. Should be fine for my needs.

Without getting into religious wars I won't be touching a PIC though.

Thanks

Charles

On Mon, Feb 12, 2024 at 6:29 PM David Burkitt <daveyboy17nz at gmail.com>
wrote:

> We used a PIC33H micro a few years ago that had a high resolution PWM that
> ran off it's own PLL from the system clock.
> Can't remember the actual part number, sorry.
> Regards
> Dave
>
> On Mon, 12 Feb 2024 at 16:22, Charles Manning <cdhmanning at gmail.com>
> wrote:
>
>> Thanks all
>>
>> I think the idea I had of trying to "overlap" two PWMs for better
>> precision would only work if I was using a predivider to divide one dwon to
>> a lower frequency. That won't work for my application as I want to drive
>> with at least 30 to 40kHz (preferably more). I think the only way I can
>> achieve a higher precision would be by going for a higher clock speed (ie.
>> a PWM driven from at least 120MHz).
>>
>> The other option might be to do some sort of dithering, but that will add
>> lower frequency ripple.
>>
>> I've been looking at various interesting micros recently. In STM32, there
>> are the newer U5 series which run up to 160MHz which should give more
>> resolution than I need as well as a few other features that look
>> interesting for motor control etc (eg. proper security to stop people
>> tampering).
>>
>> At the other end, there are a bunch of RISC V micros coming out of China.
>> 144MHz or faster, less than $1.
>>
>> THanks
>>
>> Regards
>>
>> Charles
>>
>>
>> On Mon, Feb 12, 2024 at 2:28 PM Robin Gilks <gb7ipd at gmail.com> wrote:
>>
>>> A lot will depend on the SOC or microcontroller you are using.
>>> I'm playing with an stm32f303re (Nucleo board)  at present and I see
>>> that Timer 2 is a 32 bit timer and can be clocked at 144MHz (i.e. system
>>> clock * 2).
>>> Another approach I've used for generating interesting PWM shapes is to
>>> use an SPI MOSI output being given data under DMA from memory. The above
>>> mirco can do SPI at 18Mb/s. Set the DMA  for circular operation and change
>>> the RAM data on the fly
>>> Yet another method if you are straying into the analog domain with a PSU
>>> is to use a DAC - same micro, 2 12 bit DAC outputs, can be driven by DMA
>>> and a maximum rate of 1MS/s for single bit output change (I've been using
>>> one at 1.6MS/s and it seems fine!!). Linearity and offset might be an issue
>>> with the DAC with errors of up to 4 and 12 LSBs respectively with 12 bit
>>> codes.
>>>
>>> On Sun, Feb 11, 2024 at 8:49 PM Mark Atherton <markaren1 at xtra.co.nz>
>>> wrote:
>>>
>>>> The resolution of a digital based PWM is always(?) going to be limited
>>>> by the associated highest available clock. You may be able to use
>>>> additional switched delays to increase resolution. AFAIK similar work
>>>> has been done in a similar vein by various extremely high resolution
>>>> FPGA based time-to-digital converter designs.
>>>>
>>>> However in the analogue domain using a ramp generator, a voltage source
>>>> and a comparator almost limitless resolution is available. Accuracy
>>>> however will (likely) be limited by thermal drift etc.
>>>>
>>>> What are you hoping to achieve, prey tell ??
>>>>
>>>> -M
>>>>
>>>>
>>>> On 2/11/2024 7:37 PM, Charles Manning wrote:
>>>> > Hi All
>>>> >
>>>> > I have an idea I'd like to bounce around to see what others think.
>>>> >
>>>> > I want to get a high precision PWM going to drive a buck switch mode
>>>> at
>>>> > a high enough frequency.
>>>> > Using a single PWM channel limits the precision I can get.
>>>> >
>>>> > So I was thinking: maybe there is a way I can combine two PWMs to
>>>> > generate a single high precision signal.
>>>> >
>>>> > For example, maybe one PWM can run at a slower frequency and provide
>>>> the
>>>> > "most significant bits "and another can run at a faster frequency and
>>>> > provide the least significant bits" and combine them with a flip flop
>>>> or
>>>> > such.
>>>> >
>>>> > Has anyone tried similar trickery in the past or have any other
>>>> cunning
>>>> > tricks?
>>>> >
>>>> > Thanks
>>>> >
>>>> > Charles
>>>> >
>>>> >
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