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ads7846_device

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notro edited this page Mar 7, 2014 · 4 revisions

ads7846_device is a kernel module for registering an ads7846 device.

Usage

sudo modprobe ads7846_device arguments

Arguments

  • busnum - SPI bus number (default=0)
  • cs - SPI chip select (default=1)
  • speed - SPI speed (default 2MHz)
  • irq
  • mode - SPI mode (default: SPI_MODE_0)
  • verbose - 0-2
  • model - 7843, 7845, 7846, 7873 (default: 7846)
  • vref_delay_usecs - 0 for external vref; etc
    micro seconds to delay between SPI transfers (spi_transfer.delay_usecs)
  • vref_mv - external vref value, milliVolts ads7846: if 0, use internal vref
    used for hwmon sensor code
  • keep_vref_on - set to keep vref on for differential measurements as well
    used in ads7846_setup_spi_msg(). READ_X(vref) == ADS_START | ADS_A2A1A0_d_ x | ADS_12_BIT | ADS_DFR | ADS_PD10_ADC_ON | (vref ? ADS_PD10_REF_ON : 0)
    #define ADS_PD10_REF_ON (2 << 0) /* vREF on + penirq */
  • swap_xy - swap x and y axes
  • settle_delay_usecs - Settling time of the analog signals; a function of Vcc and the capacitance on the X/Y drivers.
    If set to non-zero, two samples are taken with settle_delay us apart, and the second one is used. ~150 uSec with 0.01uF caps.
    micro seconds to delay between SPI transfers (spi_transfer.delay_usecs)
  • penirq_recheck_delay_usecs - If set to non-zero, after samples are taken this delay is applied and penirq is rechecked,
    to help avoid false events. This value is affected by the material used to build the touch layer.
    used in ads7846_report_state udelay(ts->penirq_recheck_delay_usecs);
  • x_plate_ohms - Used to calculate pressure in ads7846_report_state() (default: 400)
  • y_plate_ohms - Not used by driver
  • x_min - Minimum value for x-axis (default: 0)
  • x_max - Maximum value for x-axis (default: MAX_12BIT == 0x0FFF == 4095)
    input_set_abs_params(input_dev, ABS_X, pdata->x_min ? : 0, pdata->x_max ? : MAX_12BIT, 0, 0);
    from include/uapi/linux/input.h: Resolution for main axes (ABS_X, ABS_Y, ABS_Z) is reported in units per millimeter (units/mm)
  • y_min - Minimum value for y-axis (default: 0)
  • y_max - Maximum value for y-axis (default: MAX_12BIT == 0x0FFF == 4095)
    input_set_abs_params(input_dev, ABS_Y, pdata->y_min ? : 0, pdata->y_max ? : MAX_12BIT, 0, 0);
  • pressure_min, pressure_max - (pressure_max default: ~0 == 0xFFFF)
    input_set_abs_params(input_dev, ABS_PRESSURE, pdata->pressure_min, pdata->pressure_max, 0, 0);
  • debounce_max - max number of additional readings per sample (0,1,2)
    if filter is not defined and debounce_max is non-zero, filter will be ads7846_debounce_filter()
  • debounce_tol - tolerance used for filtering
    used in ads7846_debounce_filter
  • debounce_rep - additional consecutive good readings required after the first two
    used in ads7846_debounce_filter
  • gpio_pendown - the GPIO used to decide the pendown state if get_pendown_state == NULL
  • irq_flags - (default: IRQF_TRIGGER_FALLING)

Max SPI speed

Max SPI speed is: 125000 * (8+16+2) Hz = 125000 * 26 Hz = 3250 kHz
On the Raspberry Pi, we have two close values on this list: 1.9 MHz and 3.9 MHz
Asking for 2 MHz will rount down to neares value which i 1.935 MHz

The value 125000 probably comes from Datasheet - ELECTRICAL CHARACTERISTICS:

|-----------------------------------------------------------|
|                                 |    ADS7846E     |       |
| PARAMETER          | CONDITIONS | MIN | TYP | MAX | UNITS |
|-----------------------------------------------------------|
| Throughput Rate    |            |     |     | 125 | kHz   | 
|-----------------------------------------------------------|

Excerpt from ads7846.c

/* this driver doesn't aim at the peak continuous sample rate */
#define	SAMPLE_BITS	(8 /*cmd*/ + 16 /*sample*/ + 2 /* before, after */)

static int __devinit ads7846_probe(struct spi_device *spi)

	/* don't exceed max specified sample rate */
	if (spi->max_speed_hz > (125000 * SAMPLE_BITS)) {
		dev_dbg(&spi->dev, "f(sample) %d KHz?\n",
				(spi->max_speed_hz/SAMPLE_BITS)/1000);
		return -EINVAL;
	}

ADS7846 driver calibration

Try both finger and stylus when calibrating.

Pressure
x_plate_ohms controls pressure.
Load driver with x_plate_ohms=400.
Tip: Hook up a LED to IRQ to see when the controller detects something.

Monitor pressure events

evtest /dev/input/eventX | egrep ABS_PRESSURE

If no event is detected, lower the value by half and reload the driver until a satisfying value is found.

x_min, x_max, y_min, y_max
Load driver with the x_plate_ohm value, and without x_ and y_ values.

Monitor positioning events

evtest /dev/input/eventX | grep ABS_X
evtest /dev/input/eventX | grep ABS_Y

Press a stylus at the outher edges of the display to record the min/max x and y value.

swap_xy
Swaps the X and Y axis. Used in landscape mode.

Links

Application notes:

Inversion pathces

piwik

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