# Max31865 min\_value and max\_value not set
**URL:** https://klipper.discourse.group/t/max31865-min-value-and-max-value-not-set/10029
**Category:** General Discussion
**Created:** [August 20, 2023, 7:02pm UTC](https://klipper.discourse.group/t/max31865-min-value-and-max-value-not-set/10029 "2023-08-20T19:02:59Z")
**Posts on this page:** 11
**Page:** 1
### Author:  [@YoctoByte](https://klipper.discourse.group/u/YoctoByte)
#### Post date: [August 20, 2023, 7:02pm UTC](https://klipper.discourse.group/t/max31865-min-value-and-max-value-not-set/10029/1 "2023-08-20T19:02:59Z")
I’m not sure if this is the correct place to post this. Opening a github issue does not seem to be possible.
This minimal config does not work;
```auto
[mcu]
serial:/dev/serial/by-id/usb-Klipper_rp2040_...
[printer]
kinematics:none
max_accel:1
max_velocity:1
[temperature_sensor test]
sensor_type:MAX31865
sensor_pin:gpio1
spi_bus:spi0a
rtd_nominal_r:1000
rtd_reference_r:2200
rtd_num_of_wires:4
```
The problem is that for this configuration `min_value` and `max_value` are not set for the max38165 initialization on the rp2040 (both set to 0). This triggers a mcu shutdown because the reported value by the max31865 is bigger than max\_value.
Commenting out the min\_value and max\_value checks in `src/thermocouple.c` fixed the problem. But I guess the issue should be fixed somewhere in the klippy code.
Edit:
This is the relevant code in `src/thermocouple.c`;
```auto
/* check the result and stop if below or above allowed range */
if (fault || value < spi->min_value || value > spi->max_value) {
spi->invalid_count++;
if (spi->invalid_count < spi->max_invalid)
return;
try_shutdown("Thermocouple reader fault");
}
```
The error message is not really helpful for identifying the problem. I had to use the source code to find out what was happening.
---
### Author:  [@Sineos](https://klipper.discourse.group/u/Sineos)
#### Post date: [August 20, 2023, 7:17pm UTC](https://klipper.discourse.group/t/max31865-min-value-and-max-value-not-set/10029/2 "2023-08-20T19:17:31Z")
Not sure, I understand the issue. Why don’t you just specify `min_value` / `max_value`?
---
### Author:  [@YoctoByte](https://klipper.discourse.group/u/YoctoByte)
#### Post date: [August 21, 2023, 7:45am UTC](https://klipper.discourse.group/t/max31865-min-value-and-max-value-not-set/10029/3 "2023-08-21T07:45:37Z")
The `min_value` and `max_value` are used in the `query_thermocouple` command that is send to the mcu (see `klippy/extras/spi_temperature.py`);
```auto
self.mcu.add_config_cmd(
"query_thermocouple oid=%u clock=%u rest_ticks=%u"
" min_value=%u max_value=%u max_invalid_count=%u" % (
self.oid, clock, self._report_clock,
self.min_sample_value, self.max_sample_value,
MAX_INVALID_COUNT), is_init=True)
```
`min_sample_value` and `max_sample_value` are internal to the `SensorBase` class and cannot be set in the klipper config directly. Somehow these values are kept at zero for the minimal config I posted.
---
### Author:  [@Sineos](https://klipper.discourse.group/u/Sineos)
#### Post date: [August 21, 2023, 9:28am UTC](https://klipper.discourse.group/t/max31865-min-value-and-max-value-not-set/10029/4 "2023-08-21T09:28:20Z")
I still don’t get it.
Temperature sensors are applicable to following config sections:
- `[extruder]`
- `[heater_bed]`
- `[heater_generic]`
- `[temperature_sensor]`
Each of these section require a `min_value` / `max_value` as mandatory config item. One of these section without a `min_value` / `max_value` is incomplete and thus not valid. There is nothing like a “stand alone” sensor.
Also all error checking and safety mechanism rely on these values.
For me it looks like as “by design”.
---
### Author:  [@YoctoByte](https://klipper.discourse.group/u/YoctoByte)
#### Post date: [August 21, 2023, 10:32am UTC](https://klipper.discourse.group/t/max31865-min-value-and-max-value-not-set/10029/5 "2023-08-21T10:32:04Z")
Thanks for pointing me to these config sections!
Actually, `min_temp` and `max_temp` are optional for the `[temperature_sensor]` section.
In `klippy/extras/temperatur_sensor.py` I found the following code in the ` __init__ `;
```auto
self.min_temp = config.getfloat('min_temp', KELVIN_TO_CELSIUS, minval=KELVIN_TO_CELSIUS)
self.max_temp = config.getfloat('max_temp', 99999999.9, above=self.min_temp)
self.sensor.setup_minmax(self.min_temp, self.max_temp)
```
So by default the values are set to -273.15 and 9999999.9.
### MAX31865 implementation
In `klipper/extras/spi_temperature` class `SensorBase`;
```auto
def setup_minmax(self, min_temp, max_temp):
adc_range = [self.calc_adc(min_temp), self.calc_adc(max_temp)]
self.min_sample_value = min(adc_range)
self.max_sample_value = max(adc_range)
```
In `klipper/extras/spi_temperature` class `MAX31865`;
```auto
def calc_adc(self, temp):
# Calculate relative resistance via Callendar-Van Dusen formula:
# resistance = rtd_nominal_r * (1 + CVD_A * temp + CVD_B * temp**2)
R_div_nominal = 1. + CVD_A * temp + CVD_B * temp * temp
adc = int(R_div_nominal / self.adc_to_resist_div_nominal + 0.5)
adc = max(0, min(MAX31865_ADC_MAX, adc))
adc = adc << 1 # Add fault bit
```
So it did the adc calculation for -273.15 and 9999999.9 and in both cases `int(R_div_nominal / self.adc_to_resist_div_nominal + 0.5)` results in a negative number. This is than set to 0 on the next line.
### Conclusion
Even though `min_temp` and `max_temp` are optional, the default values do not work with the MAX31865.
Maybe the `calc_adc` function should be updated to return `65534` (max value reported by max31865) if it detects temp==99999999.9? (Kind of ugly off course. Breaks if default value changes.)
Or always return `65534` in case of an overflow. (probably undesired)
Or maybe the default value should be None? But than `setup_minmax` for all sensor types should be updated.
Anyway, thanks for pointing me to `min_temp` and `max_temp`! Adding that to my config will probably fix the issue (cannot test it right now).
---
### Author:  [@Sineos](https://klipper.discourse.group/u/Sineos)
#### Post date: [August 21, 2023, 10:40am UTC](https://klipper.discourse.group/t/max31865-min-value-and-max-value-not-set/10029/6 "2023-08-21T10:40:58Z")
I agree, when comparing with the code, Klipper could be more precise.
Nevertheless, according to the documentation these are mandatory values and when properly set, I do not see a “real world” problem.
---
### Author:  [@YoctoByte](https://klipper.discourse.group/u/YoctoByte)
#### Post date: [August 21, 2023, 11:08am UTC](https://klipper.discourse.group/t/max31865-min-value-and-max-value-not-set/10029/7 "2023-08-21T11:08:25Z")
One more possible fix;
The Callendar-Van Dusen formula becomes negative at 1.7 million °C. So setting the default max\_temp from 99999999.9 to “only” 1000000 would also fix the problem. I think this would be a pretty decent solution since klipper will probably never be used for nuclear fusion setups.
---
### Author:  [@Sineos](https://klipper.discourse.group/u/Sineos)
#### Post date: [August 21, 2023, 11:11am UTC](https://klipper.discourse.group/t/max31865-min-value-and-max-value-not-set/10029/8 "2023-08-21T11:11:07Z")
IMO, there is nothing to fix, but I refer it to @koconnor for final judgement.
---
### Author:  [@YoctoByte](https://klipper.discourse.group/u/YoctoByte)
#### Post date: [August 21, 2023, 12:00pm UTC](https://klipper.discourse.group/t/max31865-min-value-and-max-value-not-set/10029/9 "2023-08-21T12:00:48Z")
I made a pull request: [Limit maximum temperature in MAX31865.calc\_adc() to melting point of platinum by DavidvtWout · Pull Request #6320 · Klipper3d/klipper · GitHub](https://github.com/Klipper3d/klipper/pull/6320)
---
### Author:  [@koconnor](https://klipper.discourse.group/u/koconnor)
#### Post date: [August 21, 2023, 6:53pm UTC](https://klipper.discourse.group/t/max31865-min-value-and-max-value-not-set/10029/10 "2023-08-21T18:53:29Z")
Thanks. The proposed change seems fine to me.
The intent is for heaters to require a min\_temp/max\_temp specified. However, the intent for a `temperature_sensor` was to not require an explicit min\_temp/max\_temp. That is, it was felt that a heater would need a min/max setting to ensure safety, but it was not felt that a simple temperature sensor by itself would require that safety check.
Cheers,
-Kevin
---
### Author:  [@EddyMI3D](https://klipper.discourse.group/u/EddyMI3D)
#### Post date: [May 14, 2024, 6:53pm UTC](https://klipper.discourse.group/t/max31865-min-value-and-max-value-not-set/10029/11 "2024-05-14T18:53:25Z")