quck help on PDM DAC driver

quck help on PDM DAC driver

[Naidu Tellapati]

Hi Thierry and all,

Many thanks in advance. My name is Naidu Tellapati. I am from India.

The Pulse Density Modulation DAC which we have on our SOC produces a
form of analogue output according to the relative density of output
pulses to the intended analogue signal amplitude. The 4 PDM outputs
are provided that can be used to control targets such as an LCD
backlight. After going through Linux PWM documentation, I understand
that our PDM DAC driver does not exactly fit into the Linux PWM
framework.

The PDM DAC block takes only input clock and a 12 bit value (as input
pulse) configured in one of the SOC configuration registers as an
input and based on some simple internal logic, the block produces a
form of analogue output according to the relative density of output
pulses to the intended analogue signal amplitude. The PDM DAC block
won't take duty_cycle as input configuration parameters. Instead it
takes 12 bit value (input pulse) as a configuration parameter.

The plan is to develop a driver which could be upstream-able to Linux
Kernel. Now could you please suggest me under which linux driver
subsytem I can place my PDM Driver.

Thanks and regards,
Naidu.

Re: quck help on PDM DAC driver

[Thierry Reding]

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On Sun, Sep 21, 2014 at 04:15:42PM +0530, Naidu Tellapati wrote:
> Hi Thierry and all,
> 
> Many thanks in advance. My name is Naidu Tellapati. I am from India.
> 
> The Pulse Density Modulation DAC which we have on our SOC produces a
> form of analogue output according to the relative density of output
> pulses to the intended analogue signal amplitude. The 4 PDM outputs
> are provided that can be used to control targets such as an LCD
> backlight. After going through Linux PWM documentation, I understand
> that our PDM DAC driver does not exactly fit into the Linux PWM
> framework.
> 
> The PDM DAC block takes only input clock and a 12 bit value (as input
> pulse) configured in one of the SOC configuration registers as an
> input and based on some simple internal logic, the block produces a
> form of analogue output according to the relative density of output
> pulses to the intended analogue signal amplitude. The PDM DAC block
> won't take duty_cycle as input configuration parameters. Instead it
> takes 12 bit value (input pulse) as a configuration parameter.

What does this 12-bit value represent? It sounds to me like it's exactly
what the duty-cycle is for other devices. And if it's the only parameter
then it seems to me like the device simply has a fixed period.

Both of the above should be possible to do in the current PWM framework.
For example, you could compute the period (in ns) of the PDM based on
the input clock frequency and inter/extrapolate the 12-bit parameter to
range [0..period].

You can also add checks to refuse setting a period in pwm_config() that
doesn't correspond to the fixed period derived from the input clock.

Thierry

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