Re: [PATCH v2 3/3] iio: adc: ltc2497: add 2x conversion speed mode

On Thu, 13 Aug 2026 19:01:37 +0300
Andrei Stancovici <[email protected]> wrote:

> The LTC2499 supports a 2x output rate (SPD bit in the second
> configuration byte).  In 2x mode the offset auto-calibration is
> disabled, roughly doubling the conversion rate (~13.6Hz vs ~6.8Hz in
> simultaneous 50/60Hz rejection) while leaving linearity and full-scale
> errors unchanged (datasheet).  During a temperature measurement the part
> always converts at 1x regardless of SPD.
> 
> Expose the rate through the standard sampling_frequency /
> sampling_frequency_available ABI on the voltage channels only: SPD is
> ignored for temperature conversions, so the temperature channel
> deliberately carries no SAMP_FREQ attribute.  A new has_speed_mode
> capability flag gates the feature (LTC2499); the two-byte command path
> is now taken for has_temp || has_speed_mode, since both features need the
> second config byte.
> 
> The conversion-time wait becomes mode dependent: 150ms at 1x, 76ms at 2x
> (datasheet t_CONV max, simultaneous rejection, rounded up).  The wait is
> keyed on the conversion currently in flight, whose duration is fixed by
> the mode that was active when it started - not by the newly selected
> mode.  This matters on a 1x->2x switch: a 1x conversion may still be
> running when the first 2x read arrives, and reprogramming the device
> before it finishes would be NACKed with -EIO.  Timing is centralized in
> ltc2497core_conv_time_ms() so a future FA/FB rejection-mode selection
> can extend it into a [rejection][speed] lookup without touching callers.
> 
> LTC2496/LTC2497 (no speed mode) keep the single-byte path and the
> unchanged 150ms wait.
> 
> Validated on a live LTC2499: 20 reads take ~3.1s at 1x and ~1.6s at 2x
> (~0.5x, no -EIO), voltage and temperature readings stay sane in both
> modes, and the temperature/voltage interleave (sticky-PTAT) regression
> still passes at 1x and 2x.
> 
> Signed-off-by: Andrei Stancovici <[email protected]>
This looks fine to me

J