Re: [PATCH v2 1/2] iio: adc: Determine sampling frequencies by using minimum sample time

[Stefan Agner]

On 2015-06-24 10:33, Sanchayan Maity wrote:
> The driver currently does not take into account the minimum sample time
> as per the Figure 6-8 Chapter 9.1.1 12-bit ADC electrical characteristics.
> We set a static amount of cycles instead of considering the sample time
> as a given value, which depends on hardware characteristics.
> 
> Determine sampling frequencies by first reading the device tree property
> node and then calculating the required Long Sample Time Adder (LSTAdder)
> value based on the ADC clock frequency and sample time value obtained
> from the device tree, this LSTAdder value is then used for calculating
> the sampling frequencies possible.
> 
> Signed-off-by: Sanchayan Maity <maitysanchayan@xxxxxxxxx>

Not respecting the sampling time leads to issues especially when using
multiple channel: You basically still have some capacity from the last
measurement, which distorts the next measurements. We noticed this when
using the touch screen while measuring the SoC temperature.

The required sampling time is partly given by the SoC, but depends on
the connected analog source too. A device tree property is the right
approach here.

Acked-by: Stefan Agner <stefan@xxxxxxxx>


> ---
>  .../devicetree/bindings/iio/adc/vf610-adc.txt      |  6 ++
>  drivers/iio/adc/vf610_adc.c                        | 74 ++++++++++++++++++++--
>  2 files changed, 76 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/devicetree/bindings/iio/adc/vf610-adc.txt
> b/Documentation/devicetree/bindings/iio/adc/vf610-adc.txt
> index 3eb40e2..39b86ea 100644
> --- a/Documentation/devicetree/bindings/iio/adc/vf610-adc.txt
> +++ b/Documentation/devicetree/bindings/iio/adc/vf610-adc.txt
> @@ -17,6 +17,11 @@ Recommended properties:
>    - Frequency in normal mode (ADLPC=0, ADHSC=0)
>    - Frequency in high-speed mode (ADLPC=0, ADHSC=1)
>    - Frequency in low-power mode (ADLPC=1, ADHSC=0)
> +  - min-sample-time: Minimum sampling time in nanoseconds. This value has
> +    to be chosen according to the conversion mode and the connected analog
> +	 source resistance (R_as) and capacitance (C_as). Refer the datasheet's
> +	 operating requirements. A safe default across a wide range of R_as and
> +	 C_as as well as conversion modes is 1000ns.
>  
>  Example:
>  adc0: adc@4003b000 {
> @@ -27,5 +32,6 @@ adc0: adc@4003b000 {
>  	clock-names = "adc";
>  	fsl,adck-max-frequency = <30000000>, <40000000>,
>  				<20000000>;
> +	min-sample-time = <1000>;
>  	vref-supply = <&reg_vcc_3v3_mcu>;
>  };
> diff --git a/drivers/iio/adc/vf610_adc.c b/drivers/iio/adc/vf610_adc.c
> index 480f335..8322027 100644
> --- a/drivers/iio/adc/vf610_adc.c
> +++ b/drivers/iio/adc/vf610_adc.c
> @@ -68,6 +68,9 @@
>  #define VF610_ADC_CLK_DIV8		0x60
>  #define VF610_ADC_CLK_MASK		0x60
>  #define VF610_ADC_ADLSMP_LONG		0x10
> +#define VF610_ADC_ADSTS_SHORT   0x100
> +#define VF610_ADC_ADSTS_NORMAL  0x200
> +#define VF610_ADC_ADSTS_LONG    0x300
>  #define VF610_ADC_ADSTS_MASK		0x300
>  #define VF610_ADC_ADLPC_EN		0x80
>  #define VF610_ADC_ADHSC_EN		0x400
> @@ -124,6 +127,17 @@ enum conversion_mode_sel {
>  	VF610_ADC_CONV_LOW_POWER,
>  };
>  
> +enum lst_adder_sel {
> +	VF610_ADCK_CYCLES_3,
> +	VF610_ADCK_CYCLES_5,
> +	VF610_ADCK_CYCLES_7,
> +	VF610_ADCK_CYCLES_9,
> +	VF610_ADCK_CYCLES_13,
> +	VF610_ADCK_CYCLES_17,
> +	VF610_ADCK_CYCLES_21,
> +	VF610_ADCK_CYCLES_25,
> +};
> +
>  struct vf610_adc_feature {
>  	enum clk_sel	clk_sel;
>  	enum vol_ref	vol_ref;
> @@ -132,6 +146,8 @@ struct vf610_adc_feature {
>  	int	clk_div;
>  	int     sample_rate;
>  	int	res_mode;
> +	u32 lst_adder_index;
> +	u32 default_sample_time;
>  
>  	bool	calibration;
>  	bool	ovwren;
> @@ -155,11 +171,13 @@ struct vf610_adc {
>  };
>  
>  static const u32 vf610_hw_avgs[] = { 1, 4, 8, 16, 32 };
> +static const u32 vf610_lst_adder[] = { 3, 5, 7, 9, 13, 17, 21, 25 };
>  
>  static inline void vf610_adc_calculate_rates(struct vf610_adc *info)
>  {
>  	struct vf610_adc_feature *adc_feature = &info->adc_feature;
>  	unsigned long adck_rate, ipg_rate = clk_get_rate(info->clk);
> +	u32 adck_period, lst_addr_min;
>  	int divisor, i;
>  
>  	adck_rate = info->max_adck_rate[adc_feature->conv_mode];
> @@ -174,6 +192,19 @@ static inline void
> vf610_adc_calculate_rates(struct vf610_adc *info)
>  	}
>  
>  	/*
> +	 * Determine the long sample time adder value to be used based
> +	 * on the default minimum sample time provided.
> +	 */
> +	adck_period = NSEC_PER_SEC / adck_rate;
> +	lst_addr_min = adc_feature->default_sample_time / adck_period;
> +	for (i = 0; i < ARRAY_SIZE(vf610_lst_adder); i++) {
> +		if (vf610_lst_adder[i] > lst_addr_min) {
> +			adc_feature->lst_adder_index = i;
> +			break;
> +		}
> +	}
> +
> +	/*
>  	 * Calculate ADC sample frequencies
>  	 * Sample time unit is ADCK cycles. ADCK clk source is ipg clock,
>  	 * which is the same as bus clock.
> @@ -182,12 +213,13 @@ static inline void
> vf610_adc_calculate_rates(struct vf610_adc *info)
>  	 * SFCAdder: fixed to 6 ADCK cycles
>  	 * AverageNum: 1, 4, 8, 16, 32 samples for hardware average.
>  	 * BCT (Base Conversion Time): fixed to 25 ADCK cycles for 12 bit mode
> -	 * LSTAdder(Long Sample Time): fixed to 3 ADCK cycles
> +	 * LSTAdder(Long Sample Time): 3, 5, 7, 9, 13, 17, 21, 25 ADCK cycles
>  	 */
>  	adck_rate = ipg_rate / info->adc_feature.clk_div;
>  	for (i = 0; i < ARRAY_SIZE(vf610_hw_avgs); i++)
>  		info->sample_freq_avail[i] =
> -			adck_rate / (6 + vf610_hw_avgs[i] * (25 + 3));
> +			adck_rate / (6 + vf610_hw_avgs[i] *
> +			 (25 + vf610_lst_adder[adc_feature->lst_adder_index]));
>  }
>  
>  static inline void vf610_adc_cfg_init(struct vf610_adc *info)
> @@ -347,8 +379,40 @@ static void vf610_adc_sample_set(struct vf610_adc *info)
>  		break;
>  	}
>  
> -	/* Use the short sample mode */
> -	cfg_data &= ~(VF610_ADC_ADLSMP_LONG | VF610_ADC_ADSTS_MASK);
> +	/*
> +	 * Set ADLSMP and ADSTS based on the Long Sample Time Adder value
> +	 * determined.
> +	 */
> +	switch (adc_feature->lst_adder_index) {
> +	case VF610_ADCK_CYCLES_3:
> +		break;
> +	case VF610_ADCK_CYCLES_5:
> +		cfg_data |= VF610_ADC_ADSTS_SHORT;
> +		break;
> +	case VF610_ADCK_CYCLES_7:
> +		cfg_data |= VF610_ADC_ADSTS_NORMAL;
> +		break;
> +	case VF610_ADCK_CYCLES_9:
> +		cfg_data |= VF610_ADC_ADSTS_LONG;
> +		break;
> +	case VF610_ADCK_CYCLES_13:
> +		cfg_data |= VF610_ADC_ADLSMP_LONG;
> +		break;
> +	case VF610_ADCK_CYCLES_17:
> +		cfg_data |= VF610_ADC_ADLSMP_LONG;
> +		cfg_data |= VF610_ADC_ADSTS_SHORT;
> +		break;
> +	case VF610_ADCK_CYCLES_21:
> +		cfg_data |= VF610_ADC_ADLSMP_LONG;
> +		cfg_data |= VF610_ADC_ADSTS_NORMAL;
> +		break;
> +	case VF610_ADCK_CYCLES_25:
> +		cfg_data |= VF610_ADC_ADLSMP_LONG;
> +		cfg_data |= VF610_ADC_ADSTS_NORMAL;
> +		break;
> +	default:
> +		dev_err(info->dev, "error in sample time select\n");
> +	}
>  
>  	/* update hardware average selection */
>  	cfg_data &= ~VF610_ADC_AVGS_MASK;
> @@ -712,6 +776,8 @@ static int vf610_adc_probe(struct platform_device *pdev)
>  
>  	of_property_read_u32_array(pdev->dev.of_node, "fsl,adck-max-frequency",
>  			info->max_adck_rate, 3);
> +	of_property_read_u32(pdev->dev.of_node, "min-sample-time",
> +			&info->adc_feature.default_sample_time);
>  
>  	platform_set_drvdata(pdev, indio_dev);

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