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SubjectRe: [PATCH v5 2/3] iio: hx711: add delay until DOUT is ready
On Tue, 10 Jul 2018 20:19:03 +0200
Andreas Klinger <ak@it-klinger.de> wrote:

> On a system with parasitic capacitance it turned out that DOUT is not ready
> after 100 ns after PD_SCK has raised. A measurement showed almost 1000 ns
> until DOUT has reached its correct value.
>
> With this patch its now possible to wait until data is ready.
>
> The wait time should not be higher than the maximum PD_SCK high time which
> is corresponding to the datasheet 50000 ns.
>
> Signed-off-by: Andreas Klinger <ak@it-klinger.de>
Applied, thanks.

Jonathan

> ---
> drivers/iio/adc/hx711.c | 39 +++++++++++++++++++++++++++++++++++++++
> 1 file changed, 39 insertions(+)
>
> diff --git a/drivers/iio/adc/hx711.c b/drivers/iio/adc/hx711.c
> index 9430b54121e0..36b59d8957fb 100644
> --- a/drivers/iio/adc/hx711.c
> +++ b/drivers/iio/adc/hx711.c
> @@ -97,6 +97,14 @@ struct hx711_data {
> * 2x32-bit channel + 64-bit timestamp
> */
> u32 buffer[4];
> + /*
> + * delay after a rising edge on SCK until the data is ready DOUT
> + * this is dependent on the hx711 where the datasheet tells a
> + * maximum value of 100 ns
> + * but also on potential parasitic capacities on the wiring
> + */
> + u32 data_ready_delay_ns;
> + u32 clock_frequency;
> };
>
> static int hx711_cycle(struct hx711_data *hx711_data)
> @@ -110,6 +118,14 @@ static int hx711_cycle(struct hx711_data *hx711_data)
> */
> preempt_disable();
> gpiod_set_value(hx711_data->gpiod_pd_sck, 1);
> +
> + /*
> + * wait until DOUT is ready
> + * it turned out that parasitic capacities are extending the time
> + * until DOUT has reached it's value
> + */
> + ndelay(hx711_data->data_ready_delay_ns);
> +
> val = gpiod_get_value(hx711_data->gpiod_dout);
> /*
> * here we are not waiting for 0.2 us as suggested by the datasheet,
> @@ -120,6 +136,12 @@ static int hx711_cycle(struct hx711_data *hx711_data)
> gpiod_set_value(hx711_data->gpiod_pd_sck, 0);
> preempt_enable();
>
> + /*
> + * make it a square wave for addressing cases with capacitance on
> + * PC_SCK
> + */
> + ndelay(hx711_data->data_ready_delay_ns);
> +
> return val;
> }
>
> @@ -458,6 +480,7 @@ static const struct iio_chan_spec hx711_chan_spec[] = {
> static int hx711_probe(struct platform_device *pdev)
> {
> struct device *dev = &pdev->dev;
> + struct device_node *np = dev->of_node;
> struct hx711_data *hx711_data;
> struct iio_dev *indio_dev;
> int ret;
> @@ -530,6 +553,22 @@ static int hx711_probe(struct platform_device *pdev)
> hx711_data->gain_set = 128;
> hx711_data->gain_chan_a = 128;
>
> + hx711_data->clock_frequency = 400000;
> + ret = of_property_read_u32(np, "clock-frequency",
> + &hx711_data->clock_frequency);
> +
> + /*
> + * datasheet says the high level of PD_SCK has a maximum duration
> + * of 50 microseconds
> + */
> + if (hx711_data->clock_frequency < 20000) {
> + dev_warn(dev, "clock-frequency too low - assuming 400 kHz\n");
> + hx711_data->clock_frequency = 400000;
> + }
> +
> + hx711_data->data_ready_delay_ns =
> + 1000000000 / hx711_data->clock_frequency;
> +
> platform_set_drvdata(pdev, indio_dev);
>
> indio_dev->name = "hx711";

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