The new algorithm uses a 'one size fits em all' threshold, which should
be easier to understand and debug. I believe there are no regressions
compared to the old adaptive threshold algorithm. I don't remember why
I chose the old algorithm when I initially wrote the driver.
Signed-off-by: Harald Geyer <harald@xxxxxxxxx>
---
changes since V1:
* Simplify overly explicit code - no real change
drivers/iio/humidity/dht11.c | 64 +++++++++++++++++++++++++++++---------------
1 file changed, 42 insertions(+), 22 deletions(-)
diff --git a/drivers/iio/humidity/dht11.c b/drivers/iio/humidity/dht11.c
index 1ca284a..96185f8 100644
--- a/drivers/iio/humidity/dht11.c
+++ b/drivers/iio/humidity/dht11.c
@@ -50,12 +50,32 @@
#define DHT11_EDGES_PER_READ (2 * DHT11_BITS_PER_READ + \
DHT11_EDGES_PREAMBLE + 1)
-/* Data transmission timing (nano seconds) */
+/*
+ * Data transmission timing:
+ * Data bits are encoded as pulse length (high time) on the data line.
+ * 0-bit: 22-30uS -- typically 26uS (AM2302)
+ * 1-bit: 68-75uS -- typically 70uS (AM2302)
+ * The acutal timings also depend on the properties of the cable, with
+ * longer cables typically making pulses shorter.
+ *
+ * Our decoding depends on the time resolution of the system:
+ * timeres > 34uS ... don't know what a 1-tick pulse is
+ * 34uS > timeres > 30uS ... no problem (30kHz and 32kHz clocks)
+ * 30uS > timeres > 23uS ... don't know what a 2-tick pulse is
+ * timeres < 23uS ... no problem
+ *
+ * Luckily clocks in the 33-44kHz range are quite uncommon, so we can
+ * support most systems if the threshold for decoding a pulse as 1-bit
+ * is chosen carefully. If somebody really wants to support clocks around
+ * 40kHz, where this driver is most unreliable, there are two options.
+ * a) select an implementation using busy loop polling on those systems
+ * b) use the checksum to do some probabilistic decoding
+ */
#define DHT11_START_TRANSMISSION 18 /* ms */
-#define DHT11_SENSOR_RESPONSE 80000
-#define DHT11_START_BIT 50000
-#define DHT11_DATA_BIT_LOW 27000
-#define DHT11_DATA_BIT_HIGH 70000
+#define DHT11_MIN_TIMERES 34000 /* ns */
+#define DHT11_THRESHOLD 49000 /* ns */
+#define DHT11_AMBIG_LOW 23000 /* ns */
+#define DHT11_AMBIG_HIGH 30000 /* ns */
struct dht11 {
struct device *dev;
@@ -76,43 +96,39 @@ struct dht11 {
struct {s64 ts; int value; } edges[DHT11_EDGES_PER_READ];
};
-static unsigned char dht11_decode_byte(int *timing, int threshold)
+static unsigned char dht11_decode_byte(char *bits)
{
unsigned char ret = 0;
int i;
for (i = 0; i < 8; ++i) {
ret <<= 1;
- if (timing[i] >= threshold)
+ if (bits[i])
++ret;
}
return ret;
}
-static int dht11_decode(struct dht11 *dht11, int offset, int timeres)
+static int dht11_decode(struct dht11 *dht11, int offset)
{
- int i, t, timing[DHT11_BITS_PER_READ], threshold;
+ int i, t;
+ char bits[DHT11_BITS_PER_READ];
unsigned char temp_int, temp_dec, hum_int, hum_dec, checksum;
- threshold = DHT11_DATA_BIT_HIGH / timeres;
- if (DHT11_DATA_BIT_LOW / timeres + 1 >= threshold)
- pr_err("dht11: WARNING: decoding ambiguous\n");
-
- /* scale down with timeres and check validity */
for (i = 0; i < DHT11_BITS_PER_READ; ++i) {
t = dht11->edges[offset + 2 * i + 2].ts -
dht11->edges[offset + 2 * i + 1].ts;
if (!dht11->edges[offset + 2 * i + 1].value)
return -EIO; /* lost synchronisation */
- timing[i] = t / timeres;
+ bits[i] = t > DHT11_THRESHOLD;
}
- hum_int = dht11_decode_byte(timing, threshold);
- hum_dec = dht11_decode_byte(&timing[8], threshold);
- temp_int = dht11_decode_byte(&timing[16], threshold);
- temp_dec = dht11_decode_byte(&timing[24], threshold);
- checksum = dht11_decode_byte(&timing[32], threshold);
+ hum_int = dht11_decode_byte(bits);
+ hum_dec = dht11_decode_byte(&bits[8]);
+ temp_int = dht11_decode_byte(&bits[16]);
+ temp_dec = dht11_decode_byte(&bits[24]);
+ checksum = dht11_decode_byte(&bits[32]);
if (((hum_int + hum_dec + temp_int + temp_dec) & 0xff) != checksum)
return -EIO;
@@ -166,7 +182,7 @@ static int dht11_read_raw(struct iio_dev *iio_dev,
mutex_lock(&dht11->lock);
if (dht11->timestamp + DHT11_DATA_VALID_TIME < ktime_get_real_ns()) {
timeres = ktime_get_resolution_ns();
- if (DHT11_DATA_BIT_HIGH < 2 * timeres) {
+ if (timeres > DHT11_MIN_TIMERES) {
dev_err(dht11->dev, "timeresolution %dns too low\n",
timeres);
/* In theory a better clock could become available
@@ -176,6 +192,10 @@ static int dht11_read_raw(struct iio_dev *iio_dev,
ret = -EAGAIN;
goto err;
}
+ if (timeres > DHT11_AMBIG_LOW && timeres < DHT11_AMBIG_HIGH)
+ dev_warn(dht11->dev,
+ "timeresolution: %dns - decoding ambiguous\n",
+ timeres);
reinit_completion(&dht11->completion);
@@ -211,7 +231,7 @@ static int dht11_read_raw(struct iio_dev *iio_dev,
offset = DHT11_EDGES_PREAMBLE +
dht11->num_edges - DHT11_EDGES_PER_READ;
for (; offset >= 0; --offset) {
- ret = dht11_decode(dht11, offset, timeres);
+ ret = dht11_decode(dht11, offset);
if (!ret)
break;
}
--
2.1.4
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