The HTU21 offers 4 sampling frequencies: 20, 40, 70 and 120, which are
associated to an index that is used to select the right measurement
resolution and its corresponding measurement time. The current
implementation selects the measurement resolution and the temperature
measurement time properly, but it does not select the right humidity
measurement time in all cases.
In summary, the 40 and 70 humidity measurement times are swapped.
The reason for that is probably the unusual coding for the measurement
resolution. According to the datasheet, the bits [7,0] of the "user
register" are used as follows to select the bit resolution:
--------------------------------------------------
| Bit 7 | Bit 0 | RH | Temp | Trh (us) | Tt (us) |
--------------------------------------------------
| 0 | 0 | 12 | 14 | 16000 | 50000 |
--------------------------------------------------
| 0 | 1 | 8 | 12 | 3000 | 13000 |
--------------------------------------------------
| 1 | 0 | 10 | 13 | 5000 | 25000 |
--------------------------------------------------
| 1 | 1 | 11 | 11 | 8000 | 7000 |
--------------------------------------------------
*This table is available in the official datasheet, page 13/21. I have
just appended the times provided in the humidity/temperature tables,
pages 3/21, 5/21. Note that always a pair of resolutions is selected.
The sampling frequencies [20, 40, 70, 120] are assigned to a linear
index [0..3] which is then coded as follows [1]:
Index [7,0]
--------------
idx 0 0,0
idx 1 1,0
idx 2 0,1
idx 3 1,1
That is done that way because the temperature measurements are being
used as the reference for the sampling frequency (the frequencies and
the temperature measurement times are correlated), so increasing the
index always reduces the temperature measurement time and its
resolution. Therefore, the temperature measurement time array is as
simple as [50000, 25000, 13000, 7000]
On the other hand, the humidity resolution cannot follow the same
pattern because of the way it is coded in the "user register", where
both resolutions are selected at the same time. The humidity measurement
time array is the following: [16000, 3000, 5000, 8000], which defines
the following assignments:
Index [7,0] Trh
-----------------------
idx 0 0,0 16000 -> right, [0,0] selects 12 bits (Trh = 16000)
idx 1 1,0 3000 -> wrong! [1,0] selects 10 bits (Trh = 5000)
idx 2 0,1 5000 -> wrong! [0,1] selects 8 bits (Trh = 3000)
idx 3 1,1 8000 -> right, [1,1] selects 11 bits (Trh = 8000)
The times have been ordered as if idx = 1 -> [0,1] and idx = 2 -> [1,0],
which is not the case for the reason explained above.
So a simple modification is required to obtain the right humidity
measurement time array, swapping the values in the positions 1 and 2.
The right table should be the following: [16000, 5000, 3000, 8000]
Fix the humidity measurement time array with the right idex/value
coding.
[1] The actual code that makes this coding and assigns it to the current
value of the "user register" is the following:
config_reg &= 0x7E;
config_reg |= ((i & 1) << 7) + ((i & 2) >> 1);
Fixes: d574a87cc311 ("Add meas-spec sensors common part")
Signed-off-by: Javier Carrasco <[email protected]>
---
drivers/iio/common/ms_sensors/ms_sensors_i2c.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/drivers/iio/common/ms_sensors/ms_sensors_i2c.c b/drivers/iio/common/ms_sensors/ms_sensors_i2c.c
index 6633b35a94e6..9c9bc77003c7 100644
--- a/drivers/iio/common/ms_sensors/ms_sensors_i2c.c
+++ b/drivers/iio/common/ms_sensors/ms_sensors_i2c.c
@@ -15,8 +15,8 @@
/* Conversion times in us */
static const u16 ms_sensors_ht_t_conversion_time[] = { 50000, 25000,
13000, 7000 };
-static const u16 ms_sensors_ht_h_conversion_time[] = { 16000, 3000,
- 5000, 8000 };
+static const u16 ms_sensors_ht_h_conversion_time[] = { 16000, 5000,
+ 3000, 8000 };
static const u16 ms_sensors_tp_conversion_time[] = { 500, 1100, 2100,
4100, 8220, 16440 };
---
base-commit: 611da07b89fdd53f140d7b33013f255bf0ed8f34
change-id: 20231026-topic-htu21_conversion_time-9ea81e86703f
Best regards,
--
Javier Carrasco <[email protected]>
On Thu, 26 Oct 2023 17:44:49 +0200
Javier Carrasco <[email protected]> wrote:
> The HTU21 offers 4 sampling frequencies: 20, 40, 70 and 120, which are
> associated to an index that is used to select the right measurement
> resolution and its corresponding measurement time. The current
> implementation selects the measurement resolution and the temperature
> measurement time properly, but it does not select the right humidity
> measurement time in all cases.
>
> In summary, the 40 and 70 humidity measurement times are swapped.
>
> The reason for that is probably the unusual coding for the measurement
> resolution. According to the datasheet, the bits [7,0] of the "user
> register" are used as follows to select the bit resolution:
>
> --------------------------------------------------
> | Bit 7 | Bit 0 | RH | Temp | Trh (us) | Tt (us) |
> --------------------------------------------------
> | 0 | 0 | 12 | 14 | 16000 | 50000 |
> --------------------------------------------------
> | 0 | 1 | 8 | 12 | 3000 | 13000 |
> --------------------------------------------------
> | 1 | 0 | 10 | 13 | 5000 | 25000 |
> --------------------------------------------------
> | 1 | 1 | 11 | 11 | 8000 | 7000 |
> --------------------------------------------------
> *This table is available in the official datasheet, page 13/21. I have
> just appended the times provided in the humidity/temperature tables,
> pages 3/21, 5/21. Note that always a pair of resolutions is selected.
>
> The sampling frequencies [20, 40, 70, 120] are assigned to a linear
> index [0..3] which is then coded as follows [1]:
>
> Index [7,0]
> --------------
> idx 0 0,0
> idx 1 1,0
> idx 2 0,1
> idx 3 1,1
>
> That is done that way because the temperature measurements are being
> used as the reference for the sampling frequency (the frequencies and
> the temperature measurement times are correlated), so increasing the
> index always reduces the temperature measurement time and its
> resolution. Therefore, the temperature measurement time array is as
> simple as [50000, 25000, 13000, 7000]
>
> On the other hand, the humidity resolution cannot follow the same
> pattern because of the way it is coded in the "user register", where
> both resolutions are selected at the same time. The humidity measurement
> time array is the following: [16000, 3000, 5000, 8000], which defines
> the following assignments:
>
> Index [7,0] Trh
> -----------------------
> idx 0 0,0 16000 -> right, [0,0] selects 12 bits (Trh = 16000)
> idx 1 1,0 3000 -> wrong! [1,0] selects 10 bits (Trh = 5000)
> idx 2 0,1 5000 -> wrong! [0,1] selects 8 bits (Trh = 3000)
> idx 3 1,1 8000 -> right, [1,1] selects 11 bits (Trh = 8000)
>
> The times have been ordered as if idx = 1 -> [0,1] and idx = 2 -> [1,0],
> which is not the case for the reason explained above.
>
> So a simple modification is required to obtain the right humidity
> measurement time array, swapping the values in the positions 1 and 2.
>
> The right table should be the following: [16000, 5000, 3000, 8000]
>
> Fix the humidity measurement time array with the right idex/value
> coding.
>
> [1] The actual code that makes this coding and assigns it to the current
> value of the "user register" is the following:
> config_reg &= 0x7E;
> config_reg |= ((i & 1) << 7) + ((i & 2) >> 1);
>
> Fixes: d574a87cc311 ("Add meas-spec sensors common part")
> Signed-off-by: Javier Carrasco <[email protected]>
Good description. That's horrible :(
Applied to the fixes-togreg branch of iio.git and marked for stable.
Note however that I probably won't send a fixes pull request until after rc1
is out in about 3 weeks time.
Thanks,
Jonathan
> ---
> drivers/iio/common/ms_sensors/ms_sensors_i2c.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/iio/common/ms_sensors/ms_sensors_i2c.c b/drivers/iio/common/ms_sensors/ms_sensors_i2c.c
> index 6633b35a94e6..9c9bc77003c7 100644
> --- a/drivers/iio/common/ms_sensors/ms_sensors_i2c.c
> +++ b/drivers/iio/common/ms_sensors/ms_sensors_i2c.c
> @@ -15,8 +15,8 @@
> /* Conversion times in us */
> static const u16 ms_sensors_ht_t_conversion_time[] = { 50000, 25000,
> 13000, 7000 };
> -static const u16 ms_sensors_ht_h_conversion_time[] = { 16000, 3000,
> - 5000, 8000 };
> +static const u16 ms_sensors_ht_h_conversion_time[] = { 16000, 5000,
> + 3000, 8000 };
> static const u16 ms_sensors_tp_conversion_time[] = { 500, 1100, 2100,
> 4100, 8220, 16440 };
>
>
> ---
> base-commit: 611da07b89fdd53f140d7b33013f255bf0ed8f34
> change-id: 20231026-topic-htu21_conversion_time-9ea81e86703f
>
> Best regards,