mfd: ti_am335x_tscadc: Fix idle timeout value

The old timeout value was based on the assumption that the minimum values are used for the open and sample delay and no averaging is done. In fact the ADC and touchscreen driver both use an open delay of 152 cycles and averaging over 16 samples. This patch adjusts the timeout value accordingly Signed-off-by: Matthias Kaehlcke <[email protected]> Signed-off-by: Lee Jones <[email protected]>
jbg · Oct 23, 2013 · 1a54b7d · 1a54b7d
1 parent ecf67ac
commit 1a54b7d
Showing 1 changed file with 12 additions and 7 deletions.
diff --git a/include/linux/mfd/ti_am335x_tscadc.h b/include/linux/mfd/ti_am335x_tscadc.h
@@ -125,13 +125,18 @@
 #define TOTAL_CHANNELS		8
 
 /*
-* ADC runs at 3MHz, and it takes
-* 15 cycles to latch one data output.
-* Hence the idle time for ADC to
-* process one sample data would be
-* around 5 micro seconds.
-*/
-#define IDLE_TIMEOUT 5 /* microsec */
+ * time in us for processing a single channel, calculated as follows:
+ *
+ * num cycles = open delay + (sample delay + conv time) * averaging
+ *
+ * num cycles: 152 + (1 + 13) * 16 = 376
+ *
+ * clock frequency: 26MHz / 8 = 3.25MHz
+ * clock period: 1 / 3.25MHz = 308ns
+ *
+ * processing time: 376 * 308ns = 116us
+ */
+#define IDLE_TIMEOUT 116 /* microsec */
 
 #define TSCADC_CELLS		2