Hi Nick, On 06/29/2015 02:45 PM, Stevens, Nick wrote:
The mcp3021 scaling code is dividing the VDD (full-scale) value in millivolts by the A2D resolution to obtain the scaling factor. When VDD is 3300mV (the standard value) and the resolution is 12-bit (4096 divisions), the result is a scale factor of 3300/4096, which is always one. Effectively, the raw A2D reading is always being returned because no scaling is applied. This patch fixes the issue while still using only integer math by converting VDD to microvolts before dividing by resolution, and then converting back to millivolts at return. Signed-off-by: Nick Stevens <Nick.Stevens@xxxxxxxx> --- drivers/hwmon/mcp3021.c | 11 ++++++++--- 1 file changed, 8 insertions(+), 3 deletions(-) diff --git a/drivers/hwmon/mcp3021.c b/drivers/hwmon/mcp3021.c index d219c06..c3bbba2 100644 --- a/drivers/hwmon/mcp3021.c +++ b/drivers/hwmon/mcp3021.c @@ -87,10 +87,15 @@ static inline u16 volts_from_reg(struct mcp3021_data *data, u16 val) if (val == 0) return 0; - val = val * data->output_scale - data->output_scale / 2; + /* Convert VDD setting to uV and divide by resolution to get uV/bit */ + u32 uv_per_bit = (data->vdd * 1000) / ( + (1 << data->output_res) * data->output_scale); - return val * DIV_ROUND_CLOSEST(data->vdd, - (1 << data->output_res) * data->output_scale); + /* Scale raw reading by uV/bit */ + u32 uv_val = val * uv_per_bit; + + /* Convert back from uV to mV */ + return (u16)DIV_ROUND_CLOSEST(uv_val, 1000);
How about simplifying this to return DIV_ROUND_CLOSEST(val * data->vdd, (1 << data->output_res) * data->output_scale); instead ? Result is pretty much the same as far as I can see. You forgot to multiply uv_val with scale, which causes the results for MCP3021 to be wrong by a factor of 4. I have no idea why val = val * data->output_scale - data->output_scale / 2; in the original code subtracts data->output_scale / 2; that seems wrong to me. Any idea ? Thanks, Guenter _______________________________________________ lm-sensors mailing list lm-sensors@xxxxxxxxxxxxxx http://lists.lm-sensors.org/mailman/listinfo/lm-sensors
