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soundsk
Joined: 10 May 2008
Posts: 6
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| SHT11 & 16F877A - Need Help! |
 Posted: Thu May 15, 2008 5:23 am |
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Hi,
In the last couple of days i've been trying to get a Sensirion SHT11 sensor to work with a PIC 16F877A, but i'm not getting anywhere.
I've changed everything i could think of and my eyes are going to fall off their sockets due to so much googling...
Can someone give a hand? I'm desperate...
Thank You!
The CLK line is connected to pin D1 and the data line connected to D0 with a 10K pull up...
The code used is the one available at the forum's code library...
http://www.ccsinfo.com/forum/viewtopic.php?t=28564
When i power on the circuit the LCD displays fixed values of -39.9 ºC for temp and -4.6% for humidity...
I've had a look at the sample code from Sensrion and some other pieces of code i found, but so far i've had no luck...
The code i am using is the following:
Main:
| Code: | #include <16F877A.h>
#fuses HS,NOWDT,PUT,NOPROTECT,NOLVP
#use delay(clock=20000000)
#include<lcdts.c>
#include<sht.c>
void main()
{
float restemp, truehumid;
lcd_init();
sht_init();
while(1)
{
sht_rd (restemp, truehumid);
lcd_gotoxy(1,1);
printf(lcd_putc, "Temp : %3.1f %cC ", restemp, 223);
printf(lcd_putc, "\nRH : %3.1f %% ", truehumid);
delay_ms(500); //delay 500 ms between reading to prevent self heating of sensor
}
} |
sht.c:
| Code: | ///////////////////////////////////////////////////////////////////////////////
// //
// Driver file for SHT75 Temperature & Humidity Sensor //
// //
// ***** To initialise SHT75 sensor upon power up ***** //
// //
// Function : sht_init() //
// Return : none //
// //
// //
// ***** To measure and caluculate SHT75 temp & real RH ***** //
// //
// Function : sht_rd (temp, truehumid) //
// Return : temperature & true humidity in float values //
// //
///////////////////////////////////////////////////////////////////////////////
#define sht_data_pin PIN_D0
#define sht_clk_pin PIN_D1
//***** Function to alert SHT75 *****
void comstart (void)
{
output_float(sht_data_pin); //data high
output_low(sht_clk_pin); //clk low
delay_us(2);
output_high(sht_clk_pin); //clk high
delay_us(2);
output_low(sht_data_pin); //data low
delay_us(2);
output_low(sht_clk_pin); //clk low
delay_us(5);
output_high(sht_clk_pin); //clk high
delay_us(2);
output_float(sht_data_pin); //data high
delay_us(2);
output_low(sht_clk_pin); //clk low
}
//***** Function to write data to SHT75 *****
int1 comwrite (int8 iobyte)
{
int8 i, mask = 0x80;
int1 ack;
//Shift out command
delay_us(5);
for(i=0; i<8; i++)
{
output_low(sht_clk_pin); //clk low
if((iobyte & mask) > 0) output_float(sht_data_pin); //data high if MSB high
else output_low(sht_data_pin); //data low if MSB low
delay_us(2);
output_high(sht_clk_pin); //clk high
delay_us(2);
mask = mask >> 1; //shift to next bit
}
//Shift in ack
output_low(sht_clk_pin); //clk low
delay_us(2);
ack = input(sht_data_pin); //get ack bit
output_high(sht_clk_pin); //clk high
delay_us(2);
output_low(sht_clk_pin); //clk low
return(ack);
}
//***** Function to read data from SHT75 *****
int16 comread (void)
{
int8 i;
int16 iobyte = 0;
const int16 mask0 = 0x0000;
const int16 mask1 = 0x0001;
//shift in MSB data
for(i=0; i<8; i++)
{
iobyte = iobyte << 1;
output_high(sht_clk_pin); //clk high
delay_us(2);
if (input(sht_data_pin)) iobyte |= mask1; //shift in data bit
else iobyte |= mask0;
output_low(sht_clk_pin); //clk low
delay_us(2);
}
//send ack 0 bit
output_low(sht_data_pin); //data low
delay_us(2);
output_high(sht_clk_pin); //clk high
delay_us(5);
output_low(sht_clk_pin); //clk low
delay_us(2);
output_float(sht_data_pin); //data high
//shift in LSB data
for(i=0; i<8; i++)
{
iobyte = iobyte << 1;
output_high(sht_clk_pin); //clk high
delay_us(2);
if (input(sht_data_pin)) iobyte |= mask1; //shift in data bit
else iobyte |= mask0;
output_low(sht_clk_pin); //clk low
delay_us(2);
}
//send ack 1 bit
output_float(sht_data_pin); //data high
delay_us(2);
output_high(sht_clk_pin); //clk high
delay_us(5);
output_low(sht_clk_pin); //clk low
return(iobyte);
}
//***** Function to wait for SHT75 reading *****
void comwait (void)
{
int16 sht_delay;
output_float(sht_data_pin); //data high
output_low(sht_clk_pin); //clk low
delay_us(2);
for(sht_delay=0; sht_delay<30000; sht_delay++) // wait for max 300ms
{
if (!input(sht_data_pin)) break; //if sht_data_pin low, SHT75 ready
delay_us(10);
}
}
//***** Function to reset SHT75 communication *****
void comreset (void)
{
int8 i;
output_float(sht_data_pin); //data high
output_low(sht_clk_pin); //clk low
delay_us(5);
for(i=0; i<9; i++)
{
output_high(sht_clk_pin); //toggle clk 9 times
delay_us(5);
output_low(sht_clk_pin);
delay_us(5);
}
comstart();
}
//***** Function to soft reset SHT75 *****
void sht_soft_reset (void)
{
comreset(); //SHT75 communication reset
comwrite(0x1e); //send SHT75 reset command
delay_ms(15); //pause 15 ms
}
//***** Function to measure SHT75 temperature *****
int16 measuretemp (void)
{
int1 ack;
int16 iobyte;
comstart(); //alert SHT75
ack = comwrite(0x03); //send measure temp command and read ack status
if(ack == 1) return;
comwait(); //wait for SHT75 measurement to complete
iobyte = comread(); //read SHT75 temp data
return(iobyte);
}
//***** Function to measure SHT75 RH *****
int16 measurehumid (void)
{
int1 ack;
int16 iobyte;
comstart(); //alert SHT75
ack = comwrite(0x05); //send measure RH command and read ack status
if(ack == 1) return;
comwait(); //wait for SHT75 measurement to complete
iobyte = comread(); //read SHT75 temp data
return(iobyte);
}
//***** Function to calculate SHT75 temp & RH *****
void calculate_data (int16 temp, int16 humid, float & tc, float & rhlin, float & rhtrue)
{
float truehumid1, rh;
//calculate temperature reading
tc = ((float) temp * 0.01) - 40.0;
//calculate Real RH reading
rh = (float) humid;
rhlin = (rh * 0.0405) - (rh * rh * 0.0000028) - 4.0;
//calculate True RH reading
rhtrue = ((tc - 25.0) * (0.01 + (0.00008 * rh))) + rhlin;
}
//***** Function to measure & calculate SHT75 temp & RH *****
void sht_rd (float & temp, float & truehumid)
{
int16 restemp, reshumid;
float realhumid;
restemp = 0; truehumid = 0;
restemp = measuretemp(); //measure temp
reshumid = measurehumid(); //measure RH
calculate_data (restemp, reshumid, temp, realhumid, truehumid); //calculate temp & RH
}
//***** Function to initialise SHT75 on power-up *****
void sht_init (void)
{
comreset(); //reset SHT75
delay_ms(20); //delay for power-up
} |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Thu May 15, 2008 1:21 pm |
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1. Put an oscilloscope problem on the clock and data lines of the SHT11.
Do you see any activity ? Also check the Vdd pin on the SHT11.
Does it show any voltage ? The idle state of the DATA pin should be
at a high level. Is it ?
2. What voltage is the Vdd voltage of the PIC and the SHT11 ?
3. Is there a ground connection between the SHT11 and the PIC ?
4. Make sure you don't have the signal lines swapped.
5. Try adding a 10K pull-down resistor on the CLK line.
This is recommended by Sensiron. Note this is a pull-down,
not a pull-up. The DATA line should have a pull-up on it.
6. Add a 100 nF (0.1 uF) ceramic capacitor between the Vdd and Vss
pins on the SHT11. |
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soundsk
Joined: 10 May 2008
Posts: 6
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| Posted: Fri May 16, 2008 9:15 am |
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Hi!
First of all thank you for your reply...
Answering your questions:
I don't currently have access to an oscilloscope, but both the PIC and the sensor are being powered with 5V supplied by a 7805 and two caps.
VDD Pin on SHT shows 4.99V,and although not necessary, i added another 100nF cap right by the sensor between VDD and VSS.
Also added the 10K pull down on the clock line.
With the circuit powered on, the sensor is correctly supplied, and the DATA line (with pull up) shows 4.99V and the CLK line (with pull down) shows 4.10V.
Another thing, when compiling the code above, i get two warnings:
That function int16 measuretemp (void) is not void and does not return a value measuretemp.
That function int16 measurehumid (void) is not void and does not return a value measuretemp.
Any thoughts or suggestions on what the problem can be?
Thanks! |
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Ttelmah
Guest
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| Posted: Fri May 16, 2008 9:47 am |
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The error messages from the compiler, are because you have a 'return' without a value, in the middle of both the functions, while the functions are defined to return a value. In ANSI C, this defaults to returning a 'NULL' value, but CCS does not do this, so safer to return your own 'null' value when you prematurely exit like this.
Best Wishes |
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