How to read temp from DS18B20 using UART board - c++

Question

I bought board like this now i want to connect temperature sensor to this board.
How to read temperature from sensor in c or c++?
I tried to write some code but it won't work. I connect DS18B20 data cabel directly to TXD and RXD pins.
#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <unistd.h>
#include <fcntl.h>
#include <termios.h>
#include <cstring>
#include <inttypes.h>
#include <errno.h>
int
set_interface_attribs (int fd, int speed, int parity)
{
struct termios tty;
memset (&tty, 0, sizeof tty);
if (tcgetattr (fd, &tty) != 0)
{
std::cout<<"error "<<errno<<" from tcgetattr";
return -1;
}
cfsetospeed (&tty, speed);
cfsetispeed (&tty, speed);
tty.c_cflag = (tty.c_cflag & ~CSIZE) | CS8; // 8-bit chars
// disable IGNBRK for mismatched speed tests; otherwise receive break
// as \000 chars
tty.c_iflag &= ~IGNBRK; // ignore break signal
tty.c_lflag = 0; // no signaling chars, no echo,
// no canonical processing
tty.c_oflag = 0; // no remapping, no delays
tty.c_cc[VMIN] = 0; // read doesn't block
tty.c_cc[VTIME] = 5; // 0.5 seconds read timeout
tty.c_iflag &= ~(IXON | IXOFF | IXANY); // shut off xon/xoff ctrl
tty.c_cflag |= (CLOCAL | CREAD);// ignore modem controls,
// enable reading
tty.c_cflag &= ~(PARENB | PARODD); // shut off parity
tty.c_cflag |= parity;
tty.c_cflag &= ~CSTOPB;
tty.c_cflag &= ~CRTSCTS;
if (tcsetattr (fd, TCSANOW, &tty) != 0)
{
std::cout<<"error "<<errno<<" from tcsetattr";
return -1;
}
return 0;
}
void
set_blocking (int fd, int should_block)
{
struct termios tty;
memset (&tty, 0, sizeof tty);
if (tcgetattr (fd, &tty) != 0)
{
std::cout<<"error "<<errno<<" from tggetattr";
return;
}
tty.c_cc[VMIN] = should_block ? 1 : 0;
tty.c_cc[VTIME] = 5; // 0.5 seconds read timeout
if (tcsetattr (fd, TCSANOW, &tty) != 0)
std::cout<<"error "<<errno<<" setting term attributes";
}
int main()
{
char *portname = "/dev/ttyUSB0";
int tty_fd = open (portname, O_RDWR | O_NOCTTY | O_SYNC);
if (tty_fd < 0)
{
std::cout<<"error "<<errno<<" opening "<<portname<<": "<< strerror (errno);
return -1;
}
set_interface_attribs (tty_fd, B9600, 0); // set speed to 115,200 bps, 8n1 (no parity)
set_blocking (tty_fd, true);
unsigned char c = 0xCC;
if(!write(tty_fd, &c, sizeof(c)))
std::cout<<"Write error";
sleep(2);
unsigned char buffer[8];
int size;
if((size = read(tty_fd, &buffer, 8)) < 0)
std::cout<<"Error";
else
std::cout<<"CC("<<size<<")='"<<buffer<<"'";
std::cout<<"\n";
c = 0x44;
if(!write(tty_fd, &c, sizeof(c)))
std::cout<<"Write error2";
c = 0xBE;
if(!write(tty_fd, &c, sizeof(c)))
std::cout<<"Write error2";
sleep(2);
if((size = read(tty_fd, &buffer, 8)) < 0)
std::cout<<"Error";
else
std::cout<<"BE("<<size<<")='"<<buffer<<"'";
std::cout<<"\n######################\n";
close(tty_fd);
}
I got:
CC(1)='Č#'
BE(2)='#ž#'
######################
CC(1)='Č#'
BE(2)='#ž#'
######################
CC(1)='Č#'
BE(2)='#ž#'
######################
Can you help me?

Answer 1

You can't do this with any software. The DS18B20 is electrically incompatible with the board you have. The sensor uses a 1-wire, open-collector communication scheme that is completely different from the serial protocol normally used with this board. With great difficulty you might be able to do some bit-banging of the RTS/CTS signals but you need circuitry to combine them into a bidirectional open-collector signal.

Answer 2

You can hack the UART to communicate with 1 wire protocol.
Connect Rx to Tx and add 4.7 pull-up resistor
See the application note from maxim:
http://www.maximintegrated.com/en/app-notes/index.mvp/id/214

Answer 3

As pointed out by user3804701, it's indeed possible to communicate with a 1-Wire device using a UART interface, and the application note at https://www.maximintegrated.com/en/app-notes/index.mvp/id/214 contains all the information needed to make it work.
But the OP's code needs several fixes:
Each transaction with DS18B20 is made of 3 steps: initialization (also called reset), ROM command and function command, optionally followed by a data exchange
The initialization or reset step is performed by configuring the UART with 9600 bps baud rate, transmitting 0x0F, and receiving a dummy byte; then, the baud rate must be set to 115200 bps to perform the next steps
Following the reset step, data is sent to DS18B20 by writing to the UART a 0xFF byte for each data bit set to 1, and a 0x00 byte for each bit set to 0, starting from the least significant bit; for example, to send 0xAB (i.e. 10101011), one would write to the UART the sequence (FF FF 00 FF 00 FF 00 FF); for each byte written to the UART, there is a "return byte" that needs to be read from the UART and discarded
Data is received from DS18B20 by sending the 0xFF byte following the rules in the previous bullet point, but instead of discarding the "return bytes" reading from the UART a byte for each data bit, starting from the least significant bit: a 0xFF value means that the bit value is 1, otherwise the bit value is 0; for example, the sequence (00 FF FF 00 FF 00 00 FF) read from the UART means that DS18B20 sent 0x96 (i.e. 10010110)
If there is just one DS18B20 connected to the 1-Wire bus, all transactions can use "skip ROM" (byte value 0xCC) as ROM command
The first transaction to be performed is the one that triggers a temperature conversion from DS18B20, with function command 0x44
After waiting for the conversion to complete (which can take up to 750 ms), the host can perform a second transaction to read the DS18B20 scratchpad memory (function command 0xBE); the scratchpad memory is 9 bytes long and contains among other things the temperature value
So in summary the steps needed to get a temperature sample from DS18B20 are: reset, write 0xCC, write 0x44, wait for conversion, reset, write 0xCC, write 0xBE, read 9 bytes.
Example code that implements this stuff is available at https://github.com/dword1511/onewire-over-uart.