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>
diff --git a/drivers/input/touchscreen/ads7846.c b/drivers/input/touchscreen/ads7846.c
index 1c9069c..103ee6a 100644
--- a/drivers/input/touchscreen/ads7846.c
+++ b/drivers/input/touchscreen/ads7846.c
@@ -95,7 +95,7 @@
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 @@
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 @@
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 @@
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 @@
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 3387e44..a44fa7a 100644
--- a/include/linux/spi/ads7846.h
+++ b/include/linux/spi/ads7846.h
@@ -16,6 +16,14 @@
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;