Re: Anyone gotta clue? TMP36 Pi PICO.

On 29/03/2025 15:40, John R Walliker wrote:

On 29/03/2025 14:51, The Natural Philosopher wrote:

On 29/03/2025 13:34, The Natural Philosopher wrote:

>
I have a PI PICO W which I am developing code for amongst which is a TMP36 temperature sensor
The code I have reused form other thermostat projects but its suddenly giving the odd false reading.
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#define CHIP_TEMP_IO 28
#define CHIP_ADC 2
#define RELAY_IO 21
#define IN 0
#define OUT 1
#define ON 1
#define OFF 0
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void initialise_hardware()
     {
     adc_init();
     adc_gpio_init(CHIP_TEMP_IO);
     gpio_init(RELAY_IO);
     gpio_set_dir(RELAY_IO,OUT);
     }
void set_relay(int flag)
     {
     cyw43_arch_gpio_put(CYW43_WL_GPIO_LED_PIN, flag); // set the lamp
     gpio_put(RELAY_IO,flag);    //set the relay
     }
float read_temperature()
     {
     static int index;
     static int flag;  //initialised to zero at startup
     static uint32_t rawbuf[32];
     const float conversion_factor = 3.3f / (1<<12);
     float result;
     float last;
     int i;
     uint32_t raw;
     adc_select_input(CHIP_ADC);
     raw = adc_read();
//wtf?
     printf ("Raw=%0X\n",raw);
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     result=((float)raw)*conversion_factor;
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     return (100*result-54);   // the pcb warms them up by about 4 degrees
     }
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I am reading the temperature every two seconds as in
     while (1)
         {
         sleep_ms(2000);
         flag=(flag? OFF:ON);
         set_relay(flag);
         temperature = read_temperature();
         printf("Temperature is %.1f C\n",temperature);
         }
>
about one time in ten the output jumps to about 20°C less than expected e.g.
>
>
Raw=3BE
Temperature is 23.2 C
Raw=3BE
Temperature is 23.2 C
Raw=3BE
Temperature is 23.2 C
Raw=29B
Temperature is -0.3 C /************************
Raw=3C0
Temperature is 23.3 C
Raw=3C0
Temperature is 23.3 C
Raw=3BF
Temperature is 23.3 C
Raw=3BF
Temperature is 23.3 C
Raw=3BF
Temperature is 23.3 C
Raw=3C1
>
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I have added decoupling to the TMP36 supply which is fed from the ADC 3.3v supply which seems to be a steady 3.29V according to my DVM.
>
Do I have a bad PICO W? TMP 36 or what?
>
Other PICOs with the same chip don't do this...
The only difference is that this PICO is running different and a lot more wifi/server code
Is it possible that some interrupt is - er - interrupting the ADC read with dire results?
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>
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I cannot but think this is a hardware issue.

I found another TMP36 from an old project and switched it in.
No joy.
Looks like its PI related
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>
Vaclav Klaus
>

 I'm sure you have good reasons for using that device, but maybe consider
the TMP1075.  It is cheap and reasonably accurate. I2C should not be a
problem on the Pico.

I looked at that, but really the TMP36 is more than good enough usually and takes very little code to get it working.
If I were using other I2C peripherals I would probably use it

John
 

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