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iio: adc: vf610: Determine sampling frequencies by using minimum samp…
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…le time

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.

In case the sample time property is not specified through the device
tree, a safe default value of 1000ns is assumed.

Signed-off-by: Sanchayan Maity <maitysanchayan@gmail.com>
Acked-by: Stefan Agner <stefan@agner.ch>
Acked-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
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Sanchayan Maity authored and Jonathan Cameron committed Jul 20, 2015
1 parent f686a36 commit 5e9972c
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Showing 2 changed files with 80 additions and 4 deletions.
5 changes: 5 additions & 0 deletions Documentation/devicetree/bindings/iio/adc/vf610-adc.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -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 {
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79 changes: 75 additions & 4 deletions drivers/iio/adc/vf610_adc.c
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Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand Down Expand Up @@ -98,6 +101,8 @@
#define VF610_ADC_CALF 0x2
#define VF610_ADC_TIMEOUT msecs_to_jiffies(100)

#define DEFAULT_SAMPLE_TIME 1000

enum clk_sel {
VF610_ADCIOC_BUSCLK_SET,
VF610_ADCIOC_ALTCLK_SET,
Expand All @@ -124,6 +129,17 @@ 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;
Expand All @@ -132,6 +148,8 @@ 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;
Expand All @@ -155,11 +173,13 @@ 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];
Expand All @@ -173,6 +193,19 @@ static inline void vf610_adc_calculate_rates(struct vf610_adc *info)
adc_feature->clk_div = 8;
}

/*
* 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,
Expand All @@ -182,12 +215,13 @@ 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)
Expand Down Expand Up @@ -347,8 +381,40 @@ 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;
Expand Down Expand Up @@ -713,6 +779,11 @@ 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);

ret = of_property_read_u32(pdev->dev.of_node, "min-sample-time",
&info->adc_feature.default_sample_time);
if (ret)
info->adc_feature.default_sample_time = DEFAULT_SAMPLE_TIME;

platform_set_drvdata(pdev, indio_dev);

init_completion(&info->completion);
Expand Down

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