Category: Interface

Using a PIC Microcontroller to pull temperature of iButtonLink T-sense sensor.

I pulled this off the wayback machine because I lost this post in the great database loss of how ever many years ago. I’m re-posting because I’m working on an expanded version of this with an Arduino dev board and wanted a reminder of what I did five years ago. Originally posted 1/2/2015. No edits.

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I came across a stash of iButton T-sense 1-wire sensors.. so I grabbed a couple and decided to check out 1-wire.

Maxim makes a 1-wire device called the DS18B20. It’s a 9-12 bit temperature sensor with the possibility of being powered by parasitic power from the data line. This cuts the signal path down to a single DQ line and a return. A company called iButtonLink produces a nice little wrapper around this device called a T-Sense. There are a couple pieces of software out there that will allow you to hook these up to monitoring systems, I don’t have any though. These devices come with a 64-bit address code and can be daisy-chained which makes having many of these devices monitored very nice.

At first I thought, ugh.. lame I have to send, and parse 64-bit codes in a little 8 bit micro.. doesn’t sound like a ton of fun for just fooling around on a day off.. thank fully they have a “Skip ROM” feature/command which works similar to a broadcast but can only be used when you have one device on the bus. If there is one thing left in this project I might consider finishing it’d be to add the addressing in and daisy-chain a few of these.

Most of my research came from Microchip’s Application Note AN1199 though the T-Sense Users Manual also helped out including determining the wiring diagram.

For my circuit I hooked up 5VDC (but later ran it on 3.3V just fine) and the 5VDC return on pins 1 & 2. Then the DQ link and return on pins 4&5. The signaling is interesting as the 1-wire bus needs a weak pull and works with an open collector circuits. The master starts all signaling, writes and reads. The 1’s and 0’s are based on how long the master or slave sinks the DQ line. To accomplish this in the PIC microcontroller I switched the port from an output to a three state input when I needed the port to be in weak-pull up mode (which is also hand when I need to sample the port for a response from a slave). The pull up on the resistor in my circuit is 10Kohm but I’ve seen 4.7KOhm and I’m sure anywhere in the neighborhood is fine. Finally if you do some digging you’ll notice I run this in low speed mode, if I remember correctly the “high speed” mode is 4x faster. I don’t think speed of data transfer is really relevant when you’re waiting for 750ms for a temperature conversation though.

I initially started with just determining if there was a 1-wire device on the bus. If you perform a “reset” (master sinks the bus low for 480us then releases to hi-z for 70us and then performed a sample.. any (all) device(s) will sink the line slow to prove their presence…then another 410us of delay before continuing. I got this one first try.. better luck than my first time with I2C! I then wrote the code (including sampling tidbits of Microchip AN1199 code to optimize) to do an actual temperature conversion and request it (by commanding a “read scratch pad”). The device dumps all 9 bytes of it’s registers. On that note I just remembered I should mention I did NOTHING with the CRC byte.. that’s all you if you care.

My temperature conversion code looks like this: (bus control m = master or PIC, s=slave or sensor)

m RESET

s PRESENCE flag

m SKIP ROM (0xCC)

m CONV TEMP (0x44)

m HOLD DQ (Hold line high 750ms for 12bit conversion .. I am guessing we hold it high for the parasitic power supply)

m RESET

s PRESENCE flag

m SKIP ROM (0xCC)

m READ SCRATCHPAD (0xBE)

s 9 Bytes of Data

The 12-bit conversion is basically 0.0625 deg C for every bit from 0C, The LSB holds a temperature sign.

The output of my program looks like..

There are tons of details on what a “1” is an what a “0” is, the ROM code (READ ROM, MATCH ROM), changing the configuration to 9bit for 94 ms conversions over 12bit 750ms conversions. This is all stuff you can grab out of the DS18B20 specification sheet or AN1199.

I hope you get some use out of this.. I still have enough of these T-Sense modules.. maybe if someone really wants one I’ll drop it in the mail… or perhaps we can set up a big box of electronics to ship around and I can dump some in there.

Items I used to make this happen:

TAUTIC 20-pin Development Board

Microchip PIC 18F14K22

AST Breadboard Buddy Pro

AST tinyLEDx4

iButtonLink T-Sense

CAT5 breakout board (eBay?)

Microchip PICKit 3

Then miscellaneous tools, test equipment, jumpers and a breadboard.

The good stuff (my code):

.. also found here on pastebin: http://pastebin.com/HrLg1GqL

/*
 * File:   main.c
 * Author: Charles M Douvier
 * Contact at: http://iradan.com
 *
 * Created on Janurary 1, 2015
 *
 * Target Device:
 * 18F14K22 on Tautic 20 pin dev board
 *
 * Project: Maxim 1-Wire Testing
 *
 * Details of 1-wire protocol using Microchip AN1199
 * Field device http://datasheets.maximintegrated.com/en/ds/DS18B20.pdf
 * The 1-Wire Protocol is registered trade mark of Dallas/Maxim semiconductor.
 *
 * Some code was use by the AN1199 App Note Source code; I got stuck looking for a fast way of txing by bit-bang (yes never did this before)
 * The agreement below mentions a license agreement accompaning this software; There was none. I'll note where this software was used if you
 * want to re-write without the Microchip bits.
 * The Microchip licensing as follows:
 *
 *  * FileName:        1wire.c
 * Dependencies:
 * Processor:       PIC18
 * Complier:        MCC18 v3.13
 * Company:         Microchip Technology, Inc.
 *
 * Software License Agreement
 *
 * Copyright © 2004-2007 Microchip Technology Inc.  All rights reserved.
 *
 * Microchip licenses to you the right to use, copy and distribute Software
 * only when embedded on a Microchip microcontroller or digital signal
 * controller and used with a Microchip radio frequency transceiver, which
 * are integrated into your product or third party product (pursuant to the
 * sublicense terms in the accompanying license agreement).  You may NOT
 * modify or create derivative works of the Software.
 *
 *
 * You should refer to the license agreement accompanying this Software for
 * additional information regarding your rights and obligations.
 *
 * SOFTWARE AND DOCUMENTATION ARE PROVIDED “AS IS” WITHOUT WARRANTY OF ANY
 * KIND, EITHER EXPRESS OR IMPLIED, INCLUDING WITHOUT LIMITATION, ANY WARRANTY
 * OF MERCHANTABILITY, TITLE, NON-INFRINGEMENT AND FITNESS FOR A PARTICULAR
 * PURPOSE. IN NO EVENT SHALL MICROCHIP OR ITS LICENSORS BE LIABLE OR OBLIGATED
 * UNDER CONTRACT, NEGLIGENCE, STRICT LIABILITY, CONTRIBUTION, BREACH OF
 * WARRANTY, OR OTHER LEGAL EQUITABLE THEORY ANY DIRECT OR INDIRECT DAMAGES OR
 * EXPENSES INCLUDING BUT NOT LIMITED TO ANY INCIDENTAL, SPECIAL, INDIRECT,
 * PUNITIVE OR CONSEQUENTIAL DAMAGES, LOST PROFITS OR LOST DATA, COST OF
 * PROCUREMENT OF SUBSTITUTE GOODS, TECHNOLOGY, SERVICES, OR ANY CLAIMS BY
 * THIRD PARTIES (INCLUDING BUT NOT LIMITED TO ANY DEFENSE THEREOF), OR OTHER
 * SIMILAR COSTS.
 *
 *
 *
 * Version:
 * 0.1  Configuration, with reset test
 * 0.2
 *
 */
#ifndef _XTAL_FREQ
#define _XTAL_FREQ 16000000 //4Mhz FRC internal osc
#define __delay_us(x) _delay((unsigned long)((x)*(_XTAL_FREQ/4000000.0)))
#define __delay_ms(x) _delay((unsigned long)((x)*(_XTAL_FREQ/4000.0)))
#endif
#include 
#include 
#include 
#include 
//config bits
#pragma config FOSC=IRC, WDTEN=OFF, PWRTEN=OFF, MCLRE=ON, CP0=OFF, CP1=OFF, BOREN=ON
#pragma config STVREN=ON, LVP=OFF, HFOFST=OFF, IESO=OFF, FCMEN=OFF
#define _XTAL_FREQ 16000000 //defined for delay
/*
 * Variables
 */
    int     device_present;             // 1 = 1-wire device on 1-wire bus
    int     i, x, y, int_temp, an4_value;               //
    long int    decm;
    int     itxdata, txdata;            //int RS232 tx data
    char    rxbuff[10], z[1], buf[4];                 //buffer for T-sense 1-wire device
    float    temperature, f, d;
    volatile unsigned int uart_data;    // use 'volatile' qualifer as this is changed in ISR
/*
 *  Functions
 */
    void interrupt ISR() {
    if (PIR1bits.RCIF)          // see if interrupt caused by incoming data .. unused currently
    {
        uart_data = RCREG;     // read the incoming data
        PIR1bits.RCIF = 0;      // clear interrupt flag
                                //
    }
    // I left this timer interrupt if I needed it later. This is unused.
    if (PIR1bits.TMR1IF)
    {
        //T1CONbits.TMR1ON = 0;
        PIR1bits.TMR1IF = 0;
        //T1CONbits.TMR1ON = 1;
    }
}
     void __delay_10ms(unsigned char n)     //__delay functions built-in can't be used for much at this speed... so!
 {
     while (n-- != 0) {
         __delay_ms(10);
     }
 }
void uart_send (unsigned int mydata_byte) {      //bytes
    while(!TXSTAbits.TRMT);    // make sure buffer full bit is high before transmitting
    TXREG = mydata_byte;       // transmit data
}
void write_uart(const char *txt)                //strings
{
                                //this send a string to the TX buffer
                                //one character at a time
       while(*txt)
       uart_send(*txt++);
}
//This code if from Microchip but is unused currently.
void uart_send_hex_ascii(unsigned char display_data)
{
	//unsigned char temp;
	//temp = ((display_data & 0xF0)>>4);
	//if (temp <= 0x09)
	//	Putchar(temp+'0');
	//else
	//	Putchar(temp+'0'+0x07);
        //
	//temp = display_data & 0x0F;
	//if (temp <= 0x09)
	//	Putchar(temp+'0');
	//else
	//	Putchar(temp+'0'+0x07);
	//Putchar('\r');
	//Putchar('\n');
}
void serial_init(void)
{
    // calculate values of SPBRGL and SPBRGH based on the desired baud rate
    //
    // For 8 bit Async mode with BRGH=0: Desired Baud rate = Fosc/64([SPBRGH:SPBRGL]+1)
    // For 8 bit Async mode with BRGH=1: Desired Baud rate = Fosc/16([SPBRGH:SPBRGL]+1)
    TXSTAbits.BRGH=1;       // select low speed Baud Rate (see baud rate calcs below)
    TXSTAbits.TX9=0;        // select 8 data bits
    TXSTAbits.TXEN=1;     // enable transmit
    BAUDCONbits.BRG16=0;
    RCSTAbits.SPEN=1;       // serial port is enabled
    RCSTAbits.RX9=0;        // select 8 data bits
    RCSTAbits.CREN=1;       // receive enabled
    SPBRG=25;               //38,400bps-ish
                            //BRG16=0, 7=31.25k, 25=9.615k
    PIR1bits.RCIF=0;        // make sure receive interrupt flag is clear
    PIE1bits.RCIE=1;        // enable UART Receive interrupt
         __delay_ms(10);        // give time for voltage levels on board to settle
}
void init_io(void) {
    ANSEL = 0x00;         
    ANSELH = 0x00;
    TRISAbits.TRISA0 = 0; // output
    TRISAbits.TRISA1 = 0; // output
    TRISAbits.TRISA2 = 0; // output
    TRISAbits.TRISA4 = 0; // output
    TRISAbits.TRISA5 = 0; // output
    TRISBbits.TRISB4 = 0; // output
    TRISBbits.TRISB5 = 1; // input (RX UART)
    TRISBbits.TRISB6 = 0; // output
    TRISBbits.TRISB7 = 0; // output (TX UART)
    LATC = 0x00;
    TRISCbits.TRISC0 = 1; // AN4
    TRISCbits.TRISC1 = 1; // 1-wire data
    TRISCbits.TRISC2 = 0; // 
    TRISCbits.TRISC3 = 0; // 
    TRISCbits.TRISC4 = 0; // 
    TRISCbits.TRISC5 = 0; // output
    TRISCbits.TRISC6 = 1; // input
    TRISCbits.TRISC7 = 1; // input
}
void init_adc (void)
{
    ANSELbits.ANSEL4=1;         //PORTC.0
    ADCON2bits.ADCS = 0x02;     //Fosc/32
    ADCON2bits.ADFM=0;          //left oriented
    ADCON1=0x00;
}
void read_adc (void)
{
    ADCON0bits.CHS0 = 0;        // AD4
    ADCON0bits.CHS1 = 0;
    ADCON0bits.CHS2 = 1;
    ADCON0bits.CHS3 = 0;
    ADCON0bits.ADON = 1;        // A/D ON
    __delay_us(5);
    ADCON0bits.GO   = 1;        // ..GO!
    __delay_us(5);
        while (ADCON0bits.GO) continue;              //wait for conversion
        an4_value = ADRESH;                          //AN4 value
}
void one_wire_reset(void) {
    device_present = 0x00;
    TRISCbits.TRISC1 = 0;
    LATCbits.LATC1 = 0;
    __delay_us(240);        //delay 480 us
    __delay_us(240);
    TRISCbits.TRISC1 = 1;
    __delay_us(70);
    if (!PORTCbits.RC1) {
            device_present = 0x01;
    }
    __delay_us(205);        //delay 410 us
    __delay_us(205);
}
//this looks a lot like the Microchip code, it was not I just happened to be on the right track.
void one_wire_tx_bit(unsigned char txbit) {         // write a bit
    if (txbit) {
    TRISCbits.TRISC1 = 0;
    LATCbits.LATC1 = 0;
    __delay_us(6);    
    TRISCbits.TRISC1 = 1;
    __delay_us(64);      
    }
    else {      
    TRISCbits.TRISC1 = 0;
    LATCbits.LATC1 = 0;
    __delay_us(60);    
    TRISCbits.TRISC1 = 1;
    __delay_us(10);    
    }
}
//from Microchip AN1199 code, renamed and slightly modified to match my software
/**********************************************************************
* Function:        void OW_write_byte (unsigned char write_data)
* PreCondition:    None
* Input:		   Send byte to 1-wire slave device
* Output:		   None
* Overview:		   This function used to transmit a complete byte to slave device.
*				   
***********************************************************************/
void one_wire_tx_byte (unsigned char write_data)
{
	unsigned char loop;
	
	for (loop = 0; loop < 8; loop++) 	{ 		one_wire_tx_bit(write_data & 0x01); 	//Sending LS-bit first 		write_data >>= 1;					// shift the data byte for the next bit to send
	}	
}	
//from Microchip AN1199 code: I gathered the essence of this but seeing as I am not using most of the AN1199 code
//and this would not work with XC8 I had to re-write this.
/**********************************************************************
* Function:        unsigned char OW_read_bit (void)
* PreCondition:    None
* Input:		   None
* Output:		   Return the status of the OW PIN
* Overview:		   This function used to read a single bit from the slave device.
*				   
***********************************************************************/
unsigned char one_wire_rx_bit (void)
{
	unsigned char read_data; 
        read_data = 0x00;
	//reading a bit 
	TRISCbits.TRISC1 = 0;
        LATCbits.LATC1 = 0; 						// Drive the bus low
	__delay_us(6);						// delay 6 microsecond (us)
	TRISCbits.TRISC1 = 1;  						// Release the bus
	__delay_us(9);						// delay 9 microsecond (us)
        if (PORTCbits.RC1) {                                    //read 1 or 0
            read_data = 0x01;
        }
	__delay_us(55);						// delay 55 microsecond (us)	
	return read_data;
}
/**********************************************************************
* Function:        unsigned char OW_read_byte (void)
* PreCondition:    None
* Input:		   None
* Output:		   Return the read byte from slave device
* Overview:		   This function used to read a complete byte from the slave device.
*				   
***********************************************************************/
unsigned char one_wire_rx_byte (void)
{
	unsigned char loop, result=0;
	
	for (loop = 0; loop < 8; loop++)                // here we are reading 8 bits (1 byte) 	{ 		 		result >>= 1; 				// shift the result to get it ready for the next bit to receive
		if (one_wire_rx_bit())
		result |= 0x80;				// if result is one, then set MS-bit
	}
	return result;					
}	
void one_wire_conversion_pulse(void) {
    	TRISCbits.TRISC1 = 0;
        LATCbits.LATC1 = 1; 		 //For T conv we drive the DQ line high for 750ms (12bit)
	__delay_us(250);                 // delay 
	__delay_us(250);                  
        __delay_us(250);                  
	TRISCbits.TRISC1 = 1; 
        LATCbits.LATC1 = 0;             //just in case this causes problems elsewhere                              
}
int main(void) {
    init_io();
    // set up oscillator control register, using internal OSC at 16MHz.
    OSCCONbits.IRCF = 0x07; //set OSCCON IRCF bits to select OSC frequency 16MHz
    OSCCONbits.SCS = 0x02; //set the SCS bits to select internal oscillator block
    //RCONbits.IPEN = 0;          //dsiable priority levels
    INTCONbits.PEIE = 1;        // Enable peripheral interrupt
    INTCONbits.GIE = 1;         // enable global interrupt
    init_adc();                 //unused but AN4 is there if I need it
    serial_init();
    uart_send ('x');
        LATAbits.LATA0 = 0; //this is just for debugging with an LA..
        __delay_us(1);
        LATAbits.LATA0 = 1; //also confirms oscillator setup is correct.. 1us width
        __delay_us(1);
        LATAbits.LATA0 = 0;
    while (1) {
        one_wire_reset();
        if (device_present) {
            LATCbits.LATC2 = 1;             //this is a 1-wire device out there for debugging
            one_wire_tx_byte(0xCC);         //skip-rom (similar to a broadcast)
            one_wire_tx_byte(0x44);         //do a temp conversion
            one_wire_conversion_pulse();    // hold DQ line high for 750ms
            one_wire_reset();
            //add additional check here later
            one_wire_tx_byte(0xCC);         //skip-rom (similar to a broadcast)
            one_wire_tx_byte(0xBE);         //read scratch pad
            for(i = 0; i<9; i++)            //reading all 9 bytes on the T-Sense
   		rxbuff[i] = one_wire_rx_byte();
            // T-Sense
            //  Byte 0 LSB of Temp
            //  Byte 1 MSB of Temp and sign
            // LSB
            //  2^3 2^2 2^1 2^0 2^-1 2^-2 2^-3 s^-4
            // MSB
            // S S S S S 2^6 2^5 2 ^ 4
            temperature = 0;
            f = 0.0625;
            //z[0] = rxbuff[1];
            //x = atoi(z);
            x = rxbuff[1];
            if (x & 0b10000000) {
                uart_send('-');
            } else {
                uart_send('+');
            }
            
            x = x & 0b00000111;
            int_temp = 0;
            int_temp = rxbuff[0];
            if (x & 0b00000001)
                int_temp = int_temp + 0x100;
            if (x & 0b00000010)
                int_temp = int_temp + 0x200;
            if (x & 0b00000100)
                int_temp = int_temp + 0x400;
            temperature = int_temp * f;
            
            int_temp = temperature;
            itoa(z, int_temp, 10);
            write_uart(z);
            uart_send('.');
            d = temperature - int_temp;
            decm = d * 1000;
            //page 374 of XC8 user guide
            ltoa(buf,decm,10);  //long conversion to buffer
            y=strlen(buf);  //uh, adding leading zeros..
            y=3-y;      //probably a better way of doing thing
            while (y)       //first figure out how many zeros
            {
                uart_send('0');  //missed 3-string length
                y=y-1;  //then send them until done
            }   
            write_uart (buf);
        
            uart_send(0x0A);        //LF
            uart_send(0x0D);        //CR
            //temperature   float temperature
            //int_temp      interger value of temperature
            __delay_10ms(254);
        }
    }
    return (EXIT_SUCCESS);
}