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Firmware for the Sinara 8451 Thermostat
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Firmware for the Sinara 8451 Thermostat


Reproducible build with Nix

Thermostat firmware is packaged using the Nix Flakes system. Install Nix 2.4+ and enable flakes by adding experimental-features = nix-command flakes to nix.conf (e.g. ~/.config/nix/nix.conf).

Once you have Flakes enabled, you can use nix build to build the firmware.

Development environment

Clone this repository and with Nix Flakes enabled, use the following commands:

nix develop
cargo build --release

The resulting ELF file will be located under target/thumbv7em-none-eabihf/release/thermostat.

Alternatively, you can install the Rust toolchain without Nix using rustup; see the Rust manifest file pulled in flake.nix to determine which Rust version to use.


Connect SWDIO/SWCLK/RST/GND to a programmer such as ST-Link v2.1. Run OpenOCD:

openocd -f interface/stlink.cfg -f target/stm32f4x.cfg

You may need to power up the programmer before powering the device. Leave OpenOCD running. Run the GNU debugger:

gdb target/thumbv7em-none-eabihf/release/thermostat

(gdb) source openocd.gdb


There are several options for flashing Thermostat. DFU requires only a micro-USB connector, whereas OpenOCD needs a JTAG/SWD adapter.

dfu-util on Linux

  • Install the DFU USB tool (dfu-util).
  • Convert firmware from ELF to BIN: arm-none-eabi-objcopy -O binary thermostat thermostat.bin (you can skip this step if using the BIN from Hydra)
  • Connect to the Micro USB connector to Thermostat below the RJ45.
  • Add jumper to Thermostat v2.0 across 2-pin jumper adjacent to JTAG connector.
  • Cycle board power to put it in DFU update mode
  • Push firmware to flash: dfu-util -a 0 -s 0x08000000:leave -D thermostat.bin
  • Remove jumper
  • Cycle power to leave DFU update mode

st.com DfuSe tool on Windows

On a Windows machine install st.com DfuSe USB device firmware upgrade (DFU) software. link.

  • add jumper to Thermostat v2.0 across 2-pin jumper adjacent to JTAG connector
  • cycle board power to put it in DFU update mode
  • connect micro-USB to PC
  • use st.com software to upload firmware
  • remove jumper
  • cycle power to leave DFU update mode


openocd -f interface/stlink.cfg -f target/stm32f4x.cfg -c "program target/thumbv7em-none-eabihf/release/thermostat verify reset;exit"



Ethernet, IP:

Use netcat to connect to port 23/tcp (telnet)

rlwrap nc -vv 23

telnet clients send binary data after connect. Enter \n once to invalidate the first line of input.

Reading ADC input

Set report mode to on for a continuous stream of input data.

The scope of this setting is per TCP session.

TCP commands

Send commands as simple text string terminated by \n. Responses are formatted as line-delimited JSON.

Syntax Function
report Show current input
report mode Show current report mode
report mode <off/on> Set report mode
pwm Show current PWM settings
pwm <0/1> max_i_pos <amp> Set maximum positive output current
pwm <0/1> max_i_neg <amp> Set maximum negative output current
pwm <0/1> max_v <volt> Set maximum output voltage
pwm <0/1> i_set <amp> Disengage PID, set fixed output current
pwm <0/1> pid Let output current to be controlled by the PID
center <0/1> <volt> Set the MAX1968 0A-centerpoint to the specified fixed voltage
center <0/1> vref Set the MAX1968 0A-centerpoint to measure from VREF
pid Show PID configuration
pid <0/1> target <deg_celsius> Set the PID controller target temperature
pid <0/1> kp <value> Set proportional gain
pid <0/1> ki <value> Set integral gain
pid <0/1> kd <value> Set differential gain
pid <0/1> output_min <amp> Set mininum output
pid <0/1> output_max <amp> Set maximum output
s-h Show Steinhart-Hart equation parameters
s-h <0/1> <t0/b/r0> <value> Set Steinhart-Hart parameter for a channel
postfilter Show postfilter settings
postfilter <0/1> off Disable postfilter
postfilter <0/1> rate <rate> Set postfilter output data rate
load [0/1] Restore configuration for channel all/0/1 from flash
save [0/1] Save configuration for channel all/0/1 to flash
reset Reset the device
dfu Reset device and enters USB device firmware update (DFU) mode
ipv4 <X.X.X.X/L> [Y.Y.Y.Y] Configure IPv4 address, netmask length, and optional default gateway
fan Show current fan settings and sensors' measurements
fan <value> Set fan power with values from 1 to 100
fan auto Enable automatic fan speed control
fcurve <a> <b> <c> Set fan controller curve coefficients (see Fan control section)
fcurve default Set fan controller curve coefficients to defaults (see Fan control section)
hwrev Show hardware revision, and settings related to it


The firmware includes experimental support for acting as a USB-Serial peripheral. Debug logging will be sent there by default (unless build 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 when USB 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

50/60 Hz filtering

The AD7172-2 ADC on the SENS inputs supports simultaneous rejection of 50 Hz ± 1 Hz and 60 Hz ± 1 Hz (dB). Affecting sampling rate, the postfilter rate can be tuned with the postfilter command.

Postfilter rate Rejection Effective sampling rate
16.67 Hz 92 dB 8.4 Hz
20 Hz 86 dB 9.1 Hz
21.25 Hz 62 dB 10 Hz
27 Hz 47 dB 10.41 Hz

Thermo-Electric Cooling (TEC)

  • Connect TEC module device 0 to TEC0- and TEC0+.
  • Connect TEC module device 1 to TEC1- and TEC1+.
  • The GND pin is for shielding not for sinking TEC module currents.

When using a TEC module with the Thermostat, the Thermostat expects the thermal load (where the thermistor is located) to cool down with a positive software current set point, and heat up with a negative current set point.

Testing heat flow direction with a low set current is recommended before installation of the TEC module.


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
i_set Amperes (Not a limit; Open-loop mode)

Example: set the maximum voltage of channel 0 to 1.5 V.

pwm 0 max_v 1.5

Example: set the maximum negative current of channel 0 to -3 A.

pwm 0 max_i_neg 3

Example: set the maximum positive current of channel 1 to 3 A.

pwm 0 max_i_pos 3

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 i_set 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

LED indicators

Name Color Meaning
L1 Red Firmware initializing
L3 Green Closed-loop mode (PID engaged)
L4 Green Firmware busy


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 Seconds 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
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

Note: With Thermostat v2 and below, the voltage and current readouts i_tec and tec_i are disabled and null due to faulty hardware that introduces a lot of noise in the signal.

PID Tuning

The thermostat implements a PID control loop for each of the TEC channels, more details on setting up the PID control loop can be found here.

Fan control

Fan control commands are available for thermostat revisions with an integrated fan system:

  1. fan - show fan stats: fan_pwm, abs_max_tec_i, auto_mode, k_a, k_b, k_c.
  2. fan auto - enable auto speed controller mode, where fan speed is controlled by the fan curve fcurve.
  3. fan <value> - set the fan power with the value from 1 to 100 and disable auto mode. There is no way to completely disable the fan. Please note that power doesn't correlate with the actual speed linearly.
  4. fcurve <a> <b> <c> - set coefficients of the controlling curve a*x^2 + b*x + c, where x is abs_max_tec_i/MAX_TEC_I, a normalized value in range [0,1], i.e. the (linear) proportion of current output capacity used, on the channel with the largest current flow. The controlling curve is also clamped to [0,1].
  5. fcurve default - restore fan curve coefficients to defaults: a = 1.0, b = 0.0, c = 0.0.