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README: doc

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@ -88,3 +88,94 @@ with logging via semihosting.)
**Caveat:** This logging does not flush its output. Doing so would
hang indefinitely if the output is not read by the USB host. Therefore
output will be truncated once buffers are full.
## Temperature measurement
Connect the thermistor with the SENS pins of the
device. Temperature-depending resistance is measured by the AD7172
ADC. To prepare conversion to a temperature, set the Beta parameters
for the Steinhart-Hart equation.
Set the base temperature in degrees celsius for the channel 0 thermistor:
```
s-h 0 t0 20
```
Set the resistance in Ohms measured at the base temperature t0:
```
s-h 0 r0 10000
```
Set the Beta parameter:
```
s-h 0 b 3800
```
## Thermo-Electric Cooling (TEC)
### Limits
Each of the MAX1968 TEC driver has analog/PWM inputs for setting
output limits.
Use the `pwm` command to see current settings and maximum values.
| Limit | Unit | Description |
| --- | :---: | --- |
| max_v | Volts | Maximum voltage |
| max_i_pos | Amperes | Maximum positive current |
| max_i_neg | Amperes | Maximum negative current |
| | Amperes | current control |
Example: set the maximum voltage of channel 0 to 1.5 V.
```
pwm 0 max_v 1.5
```
### Open-loop mode
To manually control TEC output current, omit the limit parameter of
the `pwm` command. Doing so will disengage the PID control for that
channel.
Example: set output current of channel 0 to 0 A.
```
pwm 0 0
```
## PID-stabilized temperature control
Set the target temperature of channel 0 to 20 degrees celsius:
```
pid 0 target 20
```
Enter closed-loop mode by switching control of the TEC output current
of channel 0 to the PID algorithm:
```
pwm 0 pid
```
## Reports
Use the bare `report` command to obtain a single report. Enable
continuous reporting with `report mode on`. Reports are JSON objects
with the following keys.
| Key | Unit | Description |
| --- | :---: | --- |
| `channel` | Integer | Channel `0`, or `1` |
| `time` | Milliseconds | Temperature measurement time |
| `adc` | Volts | AD7172 input |
| `sens` | Ohms | Thermistor resistance derived from `adc` |
| `temperature` | Degrees Celsius | Steinhart-Hart conversion result derived from `sens` |
| `pid_engaged` | Boolean | `true` if in closed-loop mode |
| `i_set` | Amperes | TEC output current |
| `vref` | Volts | MAX1968 VREF (1.5 V) |
| `dac_value` | Volts | AD5680 output derived from `i_set` |
| `dac_feedback` | Volts | ADC measurement of the AD5680 output |
| `i_tec` | Volts | MAX1968 TEC current monitor |
| `tec_i` | Amperes | TEC output current feedback derived from `i_tec` |
| `tec_u_meas` | Volts | Measurement of the voltage across the TEC |
| `pid_output` | Amperes | PID control output |