Input: ads7846 - introduce sample settling delay

The ads7846 driver has support for filtering, but when the chip gets
deselected between samples this causes noise. This patch adds support
for an optional settling delay time, so that two consecutive samples
will be taken with the specified delay time apart.  This ensures that
the chip won't be deselected, so the noise won't appear.

Filtering can still be done, but will have less work to do since each
time a new sample is taken the same delay applies.

Signed-off-by: Semih Hazar <semih.hazar@indefia.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

2 files changed, 72 insertions, 1 deletions

diff --git a/drivers/input/touchscreen/ads7846.c b/drivers/input/touchscreen/ads7846.c
index 1c9069cd3bae..103ee6ad299e 100644
--- a/drivers/input/touchscreen/ads7846.c
+++ b/drivers/input/touchscreen/ads7846.c
@@ -95,7 +95,7 @@ struct ads7846 {
         u16                     dummy;          /* for the pwrdown read */
         struct ts_event         tc;
 
-        struct spi_transfer     xfer[10];
+        struct spi_transfer     xfer[18];
         struct spi_message      msg[5];
         struct spi_message      *last_msg;
         int                     msg_idx;
@@ -936,6 +936,24 @@ static int __devinit ads7846_probe(struct spi_device *spi)
         x->len = 2;
         spi_message_add_tail(x, m);
 
+        /* the first sample after switching drivers can be low quality;
+         * optionally discard it, using a second one after the signals
+         * have had enough time to stabilize.
+         */
+        if (pdata->settle_delay_usecs) {
+                x->delay_usecs = pdata->settle_delay_usecs;
+
+                x++;
+                x->tx_buf = &ts->read_y;
+                x->len = 1;
+                spi_message_add_tail(x, m);
+
+                x++;
+                x->rx_buf = &ts->tc.y;
+                x->len = 2;
+                spi_message_add_tail(x, m);
+        }
+
         m->complete = ads7846_rx_val;
         m->context = ts;
 
@@ -954,6 +972,21 @@ static int __devinit ads7846_probe(struct spi_device *spi)
         x->len = 2;
         spi_message_add_tail(x, m);
 
+        /* ... maybe discard first sample ... */
+        if (pdata->settle_delay_usecs) {
+                x->delay_usecs = pdata->settle_delay_usecs;
+
+                x++;
+                x->tx_buf = &ts->read_x;
+                x->len = 1;
+                spi_message_add_tail(x, m);
+
+                x++;
+                x->rx_buf = &ts->tc.x;
+                x->len = 2;
+                spi_message_add_tail(x, m);
+        }
+
         m->complete = ads7846_rx_val;
         m->context = ts;
 
@@ -973,6 +1006,21 @@ static int __devinit ads7846_probe(struct spi_device *spi)
                 x->len = 2;
                 spi_message_add_tail(x, m);
 
+                /* ... maybe discard first sample ... */
+                if (pdata->settle_delay_usecs) {
+                        x->delay_usecs = pdata->settle_delay_usecs;
+
+                        x++;
+                        x->tx_buf = &ts->read_z1;
+                        x->len = 1;
+                        spi_message_add_tail(x, m);
+
+                        x++;
+                        x->rx_buf = &ts->tc.z1;
+                        x->len = 2;
+                        spi_message_add_tail(x, m);
+                }
+
                 m->complete = ads7846_rx_val;
                 m->context = ts;
 
@@ -990,6 +1038,21 @@ static int __devinit ads7846_probe(struct spi_device *spi)
                 x->len = 2;
                 spi_message_add_tail(x, m);
 
+                /* ... maybe discard first sample ... */
+                if (pdata->settle_delay_usecs) {
+                        x->delay_usecs = pdata->settle_delay_usecs;
+
+                        x++;
+                        x->tx_buf = &ts->read_z2;
+                        x->len = 1;
+                        spi_message_add_tail(x, m);
+
+                        x++;
+                        x->rx_buf = &ts->tc.z2;
+                        x->len = 2;
+                        spi_message_add_tail(x, m);
+                }
+
                 m->complete = ads7846_rx_val;
                 m->context = ts;
         }
diff --git a/include/linux/spi/ads7846.h b/include/linux/spi/ads7846.h
index 3387e44dfd13..a44fa7a02bd9 100644
--- a/include/linux/spi/ads7846.h
+++ b/include/linux/spi/ads7846.h
@@ -16,6 +16,14 @@ struct ads7846_platform_data {
         u16     vref_delay_usecs;       /* 0 for external vref; etc */
         int     keep_vref_on:1;         /* set to keep vref on for differential
                                          * measurements as well */
+
+        /* 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.
+         */
+        u16     settle_delay_usecs;
+
         u16     x_plate_ohms;
         u16     y_plate_ohms;