# Thermidity

AVR MCU based battery powered thermometer and hygrometer with e-ink display.  
Focus is on stable, accurate measurements and low power consumption. 
And of course on a nice, simple display.

The project is based on [avrink](https://gitbucket.luniks.net/gitdode/avrink) and uses the following 
components:

* MCU AVR ATmega328P
* Thermistor NTC 100kΩ 0.1°C
* Humidity Sensor HIH-5030
* E-Ink display Adafruit Monochrome 2.13" 250x122
    * Driver SSD1680
    * SRAM 23K640
    * SD Card Reader (not used)

Thanks to [sprintersb](https://github.com/sprintersb) for helping me to improve the code 
and making it more efficient!

<img style="width: 1000px" src="https://static.luniks.net/GitBucket/thermidity/Thermidity-07.jpg"/>

## Accuracy

AD conversion is done with 16x oversampling, yielding 12-bit virtual resolution with the 10-bit ADC, 
provided the signal contains some noise.

Each time measurements were taken, a moving average is updated which is used to calculate and display 
temperature, humidity and battery voltage.

A precision thermistor and precision low-voltage humidity sensor are used:

| Component  | Accuracy |
|------------|---------:|
| Thermistor |  ¹±0.1°C |
| HIH-5030   |    ±3%RH |

¹Series resistor 0.1% tolerance

## Power Consumption

Estimated average power consumption is about 30µA when measuring every 32 seconds and updating the display 
once in about 5 minutes, hopefully giving anoperating time of about 3 years with 3 AAA batteries 
(1100 mAh, 6% self-discharge). 

The consumption of each component at 3.8V and 22°C is about:

| Component  | Data Sheet | Measured |
|------------|-----------:|---------:|
| ATmega328P |     ¹4.2µA |    4.5µA |
| Thermistor |        N/A |     24µA |
| HIH-5030   |     ²200µA |    211µA |
| V-Divider  |        N/A |      2µA |
| Display    |       ³N/A |     14µA |

¹VCC = 3V, power-down sleep mode  
²VCC = 3.3V  
³Display in deep sleep mode + SRAM + SD Card Reader + Bus Transceiver  

Between taking measurements, the MCU is set to power-down sleep mode with the watchdog used as wake-up source. 
Additionally, the thermistor and humidity sensor are powered off and between display updates, the display 
is set to deep sleep mode. Power consumption (measured) then is about 19µA at 3.8V for MCU and display.  

When measuring temperature, humidity and battery voltage in ADC noise reduction mode, consumption seems 
to be somewhere around 2mA for 6ms, plus a brief additional MCU awake period for updating the moving 
average with measured values. Before measuring, the sensors are powered on and given 100ms to settle, 
consuming about 240µA.

When updating the display, consumption is at around 6mA for about 3 seconds. 
Before that, the MCU has to calculate and format the average measurements and buffer the frame in SRAM.

The display is updated in fast update mode, taking 1.5s instead of 3s for full update mode, 
saving a considerable amount of power. To avoid ghosting effects, every 10th update is a full update.

When no measurement has changed, the display is not updated at all to extend its lifetime and to save more power.

Below the cutoff voltage of 3.0V, the watchdog is disabled to stop measuring andupdating the display, 
to at least delay total discharge of the batteries. Consumption of the MCU then is at about 1µA and 
19µA of the display.

The clock of unused modules TWI, all three timers and USART is switched off to reduce power consumption.

Disabling the ADC between measurements and the SPI between display updates both contributes to a significant 
reduction of power consumption. With an enabled SPI, consumption of the display including its peripheral is 
extremely unstable and varies between some tens of microamps and several milliamps. When driving the enable 
pin of the display low, consumption is stable at about 60µA. When not driving the enable pin low and disabling 
SPI (driving SCK pin low) consumption is at about 14µA.
