WEATHER STATION WITH SERIAL COMMUNICATION

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1 WEATHER STATION WITH SERIAL COMMUNICATION Written by: Wenbo Ye, Xiao Qu, Carl-Wilhelm Igelström FACULTY OF ENGINEERING, LTH Digital and Analogue Projects EITF11

2 Contents Introduction... 2 Requirements... 2 Hardware Specification... 2 Processor... 2 LCD-Display... 2 Sensor... 2 Serial communication... 3 Keypad and encoder... 3 Construction... 3 Schematic... 3 Connecting the components... 3 Programming... 3 Results... 4 Conclusion and comments... 4 References and Data sheets... 5 Attachments... 5 Circuit Schematic... 5 Source Code... 5

3 Introduction This course, EITF11, is an elementary course in building and constructing an electronic device which can be either analogue, only using basic electronic components, or digital, using a microcontroller. The students are by the end of the course present a finished and working prototype along with documentation regarding the project. We as a group chose to build a thermometer using an ATMega16 microcontroller programming it in C using Atmel s Studio4 together with a JTagICE MKII programmer. The project is given a relative short time frame for completion and thus we decided to make the most basic device possible and in case of more time over adding more features to the device. In order to be successful in this project it is was crucial to be able to read and understand the respective datasheets accompanying the used hardware, hence we spent the initial time to get accustomed to the way of reading hardware specifications. Requirements The required and implemented functions for the device was established to: Measure current temperature Display the temperature in Fahrenheit and Celsius on a LCD screen The ability to switch between displaying the temperature in Fahrenheit, Celsius and a menu through a keypad Send a data stream containing the measured temperature to a computer through serial communication Automatically update the temperature with appropriate time interval Hardware Specification The hardware components used in this project is described in this paragraph. Processor The processor is an 8-bit ATMega16 using the internal clock at 8 MHz and with a built in A/D converter. It has 32 programmable I/O ports, some of which were by default occupied by the JTagICE MkII programming interface. LCD-Display The display is a 16x2 row LCD display of an unknown brand, but functionality and datasheet corresponds to a SHARP Dot-Matrix LCD unit. Every symbol written to the display is represented by 8- bits. Sensor The temperature sensor, the LM335 gives an output voltage proportional to the temperature in Kelvin. It gives zero voltage output at zero degrees Kelvin and thus we avoid the need to implement negative reference voltages otherwise needed with the available centigrade sensor, the LM35. One degree Kelvin corresponds to an output voltage of 10mV, which is then sent to the A/D converter in the microprocessor and then inside the processor using programming convert the voltage into temperature (Kelvin) and lastly convert it into Celsius and Fahrenheit.

4 Serial communication The possibility of communicating with a computer through a serial RS-232 port was implemented using a standard RS-232 female module connected to a MAX233 serial driver. It is needed in order to convert the signals to and from the serial port into the circuit. The signals running inside the serial needs a higher voltage than inside the circuit. The MAX232 have the ability to use a normal +5V power supply and then amplify the electric tension to the ±7.5V needed in the serial port. Keypad and encoder The keypad used is a generic 4x4 keypad of an unknown brand which we used together with an encoder in order to save pins going into the processor. The encoder is of the brand MM74C922, it sends the information from out keypad to the microprocessor. Construction Schematic The schematic was drawn beforehand construction phase using a software supplied by the school, overall after the first schematic was finished we only needed to rearrange minor details. The reason for this was because of the initial time of the project was spent thoroughly studying the datasheets. Connecting the components The core of our project is ATmega16 microcontroller. The keypad is connected to the microcontroller through an encoder to Port D, therefore only 4 available pins on the microcontroller are required. The LCD display is connected directly to port B of the processor and the display s 3 control bits are connected to Port A. The temperature sensor is connected to Port A where the microprocessor is able to obtain the voltage value with an ADC converter. In order to transform voltage signal between the microcontroller and the PC, a MAX233A was required and it is connected to Port D. During the hardware implement section, a couple of problems presented themselves. Firstly we used LM35 but since it will output zero volt at zero degrees, more efforts are required to make it work due to the need of negative reference voltages for temperatures below zero. We changed the sensor to LM335 which instead outputs a voltage corresponding to a Kelvin temperature scale, which in practice changed the output in a way that the sensor instead outputs zero voltage at zero degrees Kelvin, avoiding the use of negative reference voltages. The sensor still failed to work in the beginning, more precise: It was not measuring any temperature at all. It turned out that in the datasheet, the reference drawing that shows the sensor s pins was showing the pins viewed from the bottom and not from the top. Realizing our mistake in point of view this enabled us to connect the sensor in the correct way. Another important connection that we ignored in the beginning was the AREF pin, it is needed to trigger the AD convertor inside the microprocessor. After we connected the AREF pin to Vcc the AD converter started to work and the hardware was all set. Programming Our project code is written in C using AVR studio 4. In order to implement the functions of the keypad and temperature sensor as the input and LCD_Display and USART as the output, we wrote some sub-functions of LCD, ADC, timer and USART firstly, and also a sub-function for checking the button being pressed. Then we wrote sub-functions for displaying three different screens, one for displaying the home and the other two for displaying the Celsius and Fahrenheit temperature. Finally we began to write the main function to combine each sub-function.

5 We met some problems while debugging the program. Firstly we found that the LCD did not work, so we checked the connection of the hardware by setting the port value manually and found no problem with the connection. Then we inspected the code and found some mistakes in the assignment of the bits in the I/O port. By correcting these mistakes, we made the LCD work as we wanted. Then we used the autostep to run the code but found that the _delay_ms and _delay_us functions took too long time to implement so we changed them for check_busy function. Firstly we found that the check_busy function did not work properly so we split the while loop to make it less complex, and it worked well with autostep. However when we ran the code, the pointer could not get out of the check_busy function. After some modification of the while loop, we got the code running well. Some other problems occurred for instance: when we displayed the character string array on the LCD, there would be some garbled characters at the end of the array. So we limited the length of the array and the garbled characters disappeared. We also cut off some unuseful judgments in the check_button() function in order to prevent them to take too much time in judging the button we pressed. After our elaborate work, we finally finished the code with no errors exhibited in the hardware and got the expected functions as we wished. But there are still small problems, for instance, the keypad sometimes gets stuck because the system cannot read the data of the pressed key. We think that the cause for this is faulty/glitchy wiring. Results After several initializations of different parts the system displays "Display Temp" in the first line and "1:C 0:F" in the second line on the LCD which means key 1 is for the Celsius temperature and key 0 is for the Fahrenheit temperature. Then the system goes into a while loop and checks the changes of the variable checkbutton in the check_button() function for key press. If we press the key 1 on the keypad the system will start the AD conversion to get the temperature value from the temperature sensor and go into a screen which shows the Celsius temperature in the first line and "3:Back to home" in the second line. The temperature will update itself for every 2.1 seconds. Meanwhile the value will be sent to the computer by serial communication. If we press key 3 to go back to the first screen and then press key 2, the system will show the Fahrenheit temperature and "3:Back to home" on the screen and update and send the temperature to the computer for every 2.1 seconds. Conclusion and comments The project concluded in a working thermometer with a LCD-screen interface and serial communication capabilities. Many of the problems that arose were due to lack of experience within this field. The overall understanding for how to carry out an electronics project has increased and also the ability to read and understand datasheets have increased dramatically. In general the workflow progressed surprisingly smooth and most problems were solved within a reasonable timeframe. Further addable features include more programming inside of the PC that the device is communicating with. Such as trends and possible predictions of future temperature changes, or just simply collection of temperature data. There is also a possibility to change the communication interface from RS232 to Wi-Fi, something that would be a more modern solution. Further improvements involve cosmetics, durability and addition of extra sensors related to measuring weather metrics such as air pressure.

References and Data sheets ATMega16 microprocessor: http://www.eit.lth.se/fileadmin/eit/courses/edi021/datablad/processors/atmega16.pdf LM335 Kelvin temperature sensor: http://www.eit.lth.se/fileadmin/eit/courses/edi021/datablad/sensors/lm335.

6 References and Data sheets ATMega16 microprocessor: LM335 Kelvin temperature sensor: SHARP Dot-Matrix LCD unit: 16-key Encoder: MAX232 +5V powered, Multichannel RS-232 Driver: Attachments Circuit Schematic Source Code #include <avr/io.h>

7 #include <util/delay.h> #include <math.h> #include <avr/interrupt.h> #include <avr/signal.h> #define F_CPU UL // 8 MHz #define USART_BAUDRATE 9600 // Baud Rate value #define BAUD_PRESCALE (((F_CPU / (USART_BAUDRATE * 16UL))) - 1) unsigned char flag=0,celtempchar[3],fahtempchar[4],button=0xff,flag1=0; char checkbutton; int templ,temph,temp,celtemp,fahtemp; //void _delay_ms (double ms) //void _delay_us (double us) included in delay.h //Subfunction:I/O Port initialization void IOinit() DDRA = 0x0e; DDRB = 0xff; DDRC = 0xff; DDRD = 0xfa; //set PA0,PA4-PA7 to input ADC,set PA1-PA3 to output //set PB0-PB7 to output //set PC0-PC7 to output //set PD0,PD2 to input,set others to output PORTA =0x00; PORTB =0x00; //Subfunction:write instruction to LCD void write_cmd_lcd(char cmd) PORTA &=~(1<<PA2);

8 PORTA &=~(1<<PA3); //set RS=0 and RW=0 PORTB = cmd; //enable signal and send data to PORTB PORTA =(1<<PA1); //set E=1 of LCD PORTA &=~(1<<PA1); //E=0 of LCD //Subfunction:write data to LCD void write_data_lcd(char data) PORTA &=~(1<<PA2); PORTA = (1<<PA3); //set RS=1 and RW=0 PORTB = data; PORTA =(1<<PA1); //enable signal and send data to PORTB //set E=1 of LCD PORTA &=~(1<<PA1); //E=0 of LCD //Subfunction:read data from LCD unsigned char read_data_lcd() unsigned char address; DDRB = 0x00; //set PB0-PB7 to input PORTA = (1<<PA2); PORTA &=~(1<<PA3); PORTA =(1<<PA1); //set RS=0 and RW=1 //set E=1 of LCD

9 address = PINB; //read data from PORTB PORTA &=~(1<<PA1); //E=0 of LCD DDRB = 0xff; //set PB0-PB7 to output return address; //Subfunction:read data from LCD void check_busy() unsigned char x; x=read_data_lcd()&0x80; while(x==0x80) x=read_data_lcd()&0x80; //Subfunction:LCD initialization void LCDinit() write_cmd_lcd(0x38); //function set:8bit mode write_cmd_lcd(0x07); //entry mode set write_cmd_lcd(0x01);//clear display write_cmd_lcd(0x02);//cursor home

10 write_cmd_lcd(0x0f);//display on/off //Subfunction:clear LCD void clear_lcd() write_cmd_lcd(0x01);//clear display //Subfunction:display a string on LCD void disp_string(unsigned char*s,unsigned char length) unsigned char i=0; for(i=0;i<length;i++) write_data_lcd(s[i]); //Subfunction:ADC initialization void ADInit()

11 ADMUX = 0x00; //Set Reference to AREF and input to ADC0 ADCSRA = 0b ; //Enable ADC and ADC interrupt, set prescaler to 16 SFIOR = 0x00; sei(); //Free Running mode //Enable global interrupts //Subfunction:ADC interrupt ISR(ADC_vect) templ = ADCL; temph = (ADCH & 0x03); temp = (temph*256)+templ; celtemp = temp/1023.0* ; temperature fahtemp = 9/5.0*celtemp+32; temperature //read data from the ADCL and ADCH register //temph is shifted 8 bits to get the total value of 10 bits. //change the ADC conversion result to Celsius //change the Celsius temperature to Fahrenheit ADCSRA =(1<<ADSC); //restart the AD conversion //Subfunction:USART initialization void usart_init() circuitry UCSRB = (1 << RXEN) (1 << TXEN); // Turn on the transmission and reception UCSRC = (1 << URSEL) (1 << UCSZ0) (1 << UCSZ1); sizes,1 stop bit // Use 8-bit character UBRRL = BAUD_PRESCALE; low byte of the UBRR register // Load lower 8-bits of the baud rate value into the

12 UBRRH = (BAUD_PRESCALE >> 8); // Load upper 8-bits of the baud rate value into the high byte of the UBRR register //Subfunction:USART receive function unsigned int usart_receive() while (!(UCSRA & (1 << RXC))); // Wait for data to be received return UDR; // Get and return received data from buffer //Subfunction:USART transmit function void usart_transmit( unsigned char data ) while (!( UCSRA & (1<<UDRE)) ); // Wait for empty transmit buffer UDR = data; // Put data into buffer, sends the data //Subfunction:transmit a string void tran_string(unsigned char*s,unsigned char length) unsigned char i=0; for(i=0;i<length;i++) usart_transmit(s[i]);

13 //Subfunction:Timer1 initialization void timer1_init() TIMSK =(1<<TOIE1); // enable timer overflow interrupt for Timer1 TCCR1B = (1 << CS12); //set Timer1 register with prescale value 256 sei(); //Enable global interrupts //Subfunction:Timer1 interrupt ISR(TIMER1_OVF_vect) flag1=1; //set flag1 to 1 for every 65535*255*1/8M=2.1 seconds //Subfunction:check the button pressed on the keypad void check_button() button = (PINA & 0xf0); //masks the four least significant digits of the A-port switch(button) case 0x00: checkbutton = 0; break; case 0x40: checkbutton = 1; break;

14 case 0xc0: checkbutton = 3; break; default: checkbutton = -1; break; //Subfunction:display home menu void disp_home() write_cmd_lcd(0x06); disp_string("display Temp",12); write_cmd_lcd(0xc0); disp_string("1:c 0:F",8); //Subfunction:display Celsius temperature void disp_cel() if (celtemp<0) celtemp=-celtemp; celtempchar[0]=0xb0; //if celtemp<0, output a minus sign else celtempchar[0]=0x2b; //if celtemp>0, output a plus sign

15 celtempchar[2]= celtemp%10+48; celtempchar[1]= celtemp/10%10+48; //ones digit,change integer to character variable //tens digit write_cmd_lcd(0x06); //cursor goes to the first line disp_string("temp:",5); disp_string(celtempchar,3); write_data_lcd(0xdf); write_data_lcd('c'); write_cmd_lcd(0xc0); //cursor goes to the second line disp_string("3:back to home",14); tran_string(celtempchar,3); //Subfunction:display Fahrenheit temperature void disp_fah() if (fahtemp<0) fahtemp=-fahtemp; fahtempchar[0]=0xb0; else fahtempchar[0]=0x2b; fahtempchar[3]= fahtemp%10+48; fahtempchar[2]= fahtemp/10%10+48; fahtempchar[1]= fahtemp/100%10+48; //ones digit,change integer to character variable //tens digit //hundreds digit write_cmd_lcd(0x06); disp_string("temp:",5); disp_string(fahtempchar,4); write_data_lcd(0xdf); write_data_lcd('f');

16 write_cmd_lcd(0xc0); disp_string("3:back to home",14); tran_string(fahtempchar,4); //Mainfunction void main() IOinit(); LCDinit(); ADInit(); usart_init(); timer1_init(); disp_home(); ADCSRA =(1<<ADSC); //start AD conversion while(1) check_button(); if (checkbutton==1&&flag==1&&flag1==1) //update Celsius temperature for every 2.1 seconds clear_lcd(); disp_cel(); flag1=0; else if (checkbutton==0&&flag==1&&flag1==1) //update Fahrenheit temperature for every 2.1 seconds clear_lcd(); disp_fah(); flag1=0;

17 if (checkbutton==1&&flag==0) clear_lcd(); disp_cel(); flag=1; else if (checkbutton==0&&flag==0) clear_lcd(); disp_fah(); flag=1; else if(checkbutton==3&&flag==1) clear_lcd(); disp_home(); flag=0;

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