Table of Contents

• Ambient Temperature Measurement System
  • Features
  • Design
    • System Overview
    • Inputs
    • Analog to Digital Converter
    • The Conversion Algorithm
    • Output
    • Power Saving
    • Program memory vs Data memory
    • Trade-offs
  • Conclusion
  • References

Ambient Temperature Measurement System

This project aims to engineer a system capable of measuring the temperature of the surrounding environment and displaying the result to the user.

Features

• Utilizes the MCP9700-series temperature sensor and the ATMega328p microcontroller (MCU)
• Displays temperature on two seven-segment displays
• Temperature range: 0°C to 74°C with ±1°C accuracy

Note that the firmware is exclusively programmed using AVR instruction set. Therefore, the system requires an intricate understanding of the 328P's architecture.

Design

System Overview

Figure 1: System circuitry implemented on a bread board

Figure 2: System electircal schematic with a 5V supply

Inputs

The MCP9700 is a linear thermistor integrated circuit [1]. It has 3 pins, as shown in Figure 2. The thermistor's resistance changes with temperature, which, in turn, affects the voltage at $V_{out}$. By monitoring this voltage change with the microcontroller, the temperature can be recorded. The relationship between the voltage and temperature is given by Equation (1) [1].

$$ V_{out} = T_{c} \times T_{A} + V_{0^\circ C} \ \ \ \ \ (1)$$

Where:

• $T_{c}$ is the temperature coefficient ($10mV/°C$)
• $T_{A}$ is the ambient temperature
• $V_{out}$ is the output voltage
• $V_{0^\circ C}$ is the output voltage at 0°C (50°C)

Analog to Digital Converter

To translate the voltage change at $V_{out}$ into a digital signal that the microcontroller can process, the 328P's analog to digital converter (ADC) is employed. For this project, a 10-bit resolution was chosen to detect smaller changes in temperature. The ADC clock pre-scaler was set to 128, which provides a sufficient sample rate (50kHz to 200kHz) for accurate measurements. It is important to note that the ADC's precision is enhanced, not its accuracy, which depends on the chosen reference voltage [5]. In this case, the reference voltage was set to 5V.

The Conversion Algorithm

The main logic for the assembly code is based on the flowchart in Figure 4. Two interrupts were used: an ADC conversion complete, and a Timer interrupt. The benefit of using interrupts is that the interrupt handler will only be executed when needed, thereby allowing other activities to happen in the background.

The ADC (Analog-to-Digital Converter) feature of the microcontroller allows for noise reduction when entering the idle sleep mode. During this mode, a conversion is initiated by the ADC to measure the temperature and minimize interference from other circuitry [2]. Once the conversion is finished, the ADC complete interrupt retrieves the result from the ADC registers. The obtained value is then stored in a general-purpose register.

To convert the ADC value into a temperature, a subroutine utilizes fixed-point arithmetic. Equation (2) demonstrates the conversion process, where the ADC value is multiplied by a scaling factor. The scaling factor is determined by the reference voltage (V_ref), ADC resolution, and temperature coefficient (T_c). The result is divided by the reference temperature (T_0).

$$ TA = \frac{{ADC \times \frac{{V_{ref}}}{{2^{\text{{resolution}}} \times T_c}}}}{{T_0}} = \frac{{ADC \times \frac{{5000 \ \text{{mV}}}}{{1024 \times 10 \ \text{{mV}}}} - 50}}{{T_0}} \ \ \ \ \ (2) $$

In this equation, the numerator represents the calculation of the scaling factor using specific values for V_ref, ADC resolution, and T_c. The denominator incorporates the reference temperature T_0. The resulting value represents the temperature in degrees Celsius (°C) obtained from the ADC reading.

For displaying the temperature on the seven-segment displays, the temperature value is split into its tens and units digits. This separation allows for distinct presentation of the temperature.

Equation (3) illustrates the use of the FMUL instruction to multiply the temperature value by 26 and perform a logical right shift, effectively dividing the result by 2. This calculation produces the tens value.

$$ \text{{Tens}} = (T \times 26) \times \frac{1}{2} \ \ \ \ \ (3) $$

Equation (4) determines the units value by multiplying the temperature by 10 and subtracting the previously calculated tens value.

$$ \text{{Units}} = T \times 10 - \text{{Tens}} \ \ \ \ \ (4) $$

In the above equations, the variable T represents the temperature.

Figure 4: Flow diagram showing the main logic for the code

Output

The display of both unit and tens values would typically require a total of sixteen pins, with each segment needing seven pins plus one for ground. However, to minimize the pin count, the displays are multiplexed [3]. As shown in Figure 2, this multiplexing technique reduces the required pins to seven for the segments and two for selection. To control the display, an NPN transistor is utilized as a switch. When PB0 is high, the tens value is displayed, and when PB1 is high, the units value is displayed. Additionally, the transistor amplifies a smaller current from these pins through each segment to ground [4]. To ensure proper operation, 1410-ohm resistors are connected to the transistor bases. Each segment is connected to a 470-ohm resistor instead of using a high resistance connected to the common ground of each display. This arrangement ensures that each segment draws an equal amount of current, resulting in uniform brightness. The multiplexing logic can be seen in Figure 5.

To decode the units and tens values into their corresponding binary numbers, a lookup table is stored in program memory:

;7seg truth table 	
.org 0x0100
seg:
.db 0x3f, 0x06, 0x5b, 0x4f, 0x66, 0x6d, 0x7d, 0x07, 0xff, 0x6f 

Figure 5: Multiplexing Units and Tens values to PORTD on the MCU

Timer interrupts are employed to instruct the microcontroller on which pins to set high and when to do so. This is achieved by pre-scaling the clock by a factor of 1024. With a clock speed of 16 MHz, the timer clock is reduced to approximately 16 kHz. An interrupt is triggered approximately every 8 milliseconds, corresponding to 255 ticks for Compare match A and 127 ticks for Compare match B. Figure 6 illustrates the timing diagram for both compare registers. The interrupt handler for compare registers A and B outputs the units and tens values, respectively, to PORTD.

Figure 6: Timing diagram for multiplexing interrupts

Power Saving:

Idle mode:

To minimize power consumption, the ADC is disabled when not in use [2]. Additionally, a second sleep loop is implemented to further conserve power while the ADC remains disabled.

Multiplexing:

To optimize power usage during display switching, the frequency at which the displays alternate is set as low as possible. The human eye can perceive frequencies up to approximately 60Hz [7]. Thus, the displays are turned on and off at a frequency of 120Hz, which is the lowest achievable frequency for an 8-bit timer. This ensures that the human eye cannot detect the switching, resulting in power savings.

Program memory vs Data memory:

The choice of storing the lookup table in program memory offers power-saving benefits. Program memory is non-volatile [2], meaning that the lookup table only needs to be burned into the microcontroller once. In contrast, if the lookup table were stored in data memory, it would require setup every time the device is powered up since data memory is volatile. By utilizing program memory, power consumption is reduced.

Trade-offs

While designing the final solution, multiple approaches were considered. The pros and cons of each solution are summarized in the following table:

Solution	Pros	Cons
Using a timer to start an ADC	More control over conversion start	Incompatible with sleep modes like idle mode and ADC noise cancelling mode
8-bit resolution	Fewer cycles for ADC value reading	Less precise temperature readings
Using 16 pins for displays	Easier setup	Higher power consumption due to constant display operation

In the end, the trade-offs made were good ones as more power is saved and the temperature reading is more precise.

Conclusion

The system provides a suitable temperature measurement range for in-home use, covering 0°C to 74°C with an accuracy of ±1°C. This range is considered adequate considering the MCU's maximum withstand temperature of 85°C [2] and typical room temperatures falling between 20°C and 22°C [6].

During the design and implementation process, valuable knowledge was gained on utilizing existing engineered products, understanding MCU operation, and constructing breadboard circuits. Successful aspects of the design included display multiplexing and setting up the lookup table in program memory.

To enhance the design, negative temperature readings could be included using the FMULS instruction for signed arithmetic. Additionally, the addition of another display for the fractional component of the temperature could improve accuracy to one decimal place.

In conclusion, the solution successfully displays the temperature on two seven-segment displays, incorporating the MCP9700 temperature function, ADC resolution, and reference voltage of the ATMega328p. The displayed temperature range is 0°C to 74°C with an accuracy of ±1°C. However, further improvements are possible to withstand harsher environments and provide more accurate temperature readings.

References

1. Microchip Technology Inc., "MCP9700/MCP9700A Temperature Sensor," [Online]. Available: http://ww1.microchip.com/downloads/en/DeviceDoc/20001942F.pdf.

2. Microchip Technology Inc., "ATmega48A/PA/88A/PA/168A/PA/328/P Datasheet," [Online]. Available: http://ww1.microchip.com/downloads/en/DeviceDoc/ATmega48A-PA-88A-PA-168A-PA-328-P-DS-DS40002061A.pdf.

3. A. Subero, "Programming Pic Microcontrollers with Xc8," Apress, 2018.

4. P. Horowitz and W. Hill, "The art of electronics," Cambridge Univ. Press, 1989.

5. M. Oljaca and B. Baker, "How the voltage reference affects ADC performance, Part 2," Analog Applications, pp. 5-9, 2009.

6. H. Company, "The American Heritage Dictionary Entry: Room Temperature," [Online]. Available: https://web.archive.org/web/20150108000657/https://ahdictionary.com/word/search.html?q=room+temperature [Accessed 7 June 2020].