SI7021 I2C pin setup (SOLVED)

Eric73

@robert-hh link you looking for is in tilte https://forum.pycom.io/topic/648/si7021-i2c-sensor not ?

@ttadam Have you add pysical pull up resistor on SDA and SCL to 3.3V (4.7K or 1K value) ? Even if you set software pull up to the pin definitions, when you call i2c.init i dont know if pin your specify are not setup as pure opendrain by i2c library, so perhaps a test can with pull up can be done ?

tttadam

@eric73 I am using a 1k resistor both on SDA and SCL.
The program takes much more time to run because of the i2c.scan().... But I will write ehen I have the result.

robert-hh

@eric73 @tttadam So is it this code:

from time import sleep_ms
from machine import Pin, I2C

# Default Address
SI7021_I2C_DEFAULT_ADDR = 0x40

# Commands
CMD_MEASURE_RELATIVE_HUMIDITY_HOLD_MASTER_MODE = 0xE5
CMD_MEASURE_RELATIVE_HUMIDITY = 0xF5
CMD_MEASURE_TEMPERATURE_HOLD_MASTER_MODE = 0xE3
CMD_MEASURE_TEMPERATURE = 0xF3
CMD_READ_TEMPERATURE_VALUE_FROM_PREVIOUS_RH_MEASUREMENT = 0xE0
CMD_RESET = 0xFE
CMD_WRITE_RH_T_USER_REGISTER_1 = 0xE6
CMD_READ_RH_T_USER_REGISTER_1 = 0xE7
CMD_WRITE_HEATER_CONTROL_REGISTER = 0x51
CMD_READ_HEATER_CONTROL_REGISTER = 0x11

class SI7021(object):
  def __init__(self, i2c=None):
    self.i2c = i2c
    self.addr = SI7021_I2C_DEFAULT_ADDR
    self.cbuffer = bytearray(2)
    self.cbuffer[1] = 0x00
    
  def write_command(self, command_byte):
    self.cbuffer[0] = command_byte
    self.i2c.writeto(self.addr, self.cbuffer)

  def readTemp(self):
    self.write_command(CMD_MEASURE_TEMPERATURE)
    sleep_ms(25)
    temp = self.i2c.readfrom(self.addr,3)
    temp2 = temp[0] << 8
    temp2 = temp2 | temp[1]
    return (175.72 * temp2 / 65536) - 46.85

  def readRH(self):
    self.write_command(CMD_MEASURE_RELATIVE_HUMIDITY)
    sleep_ms(25)
    rh = self.i2c.readfrom(self.addr, 3)
    rh2 = rh[0] << 8
    rh2 = rh2 | rh[1]
    return (125 * rh2 / 65536) - 6

Form the first glance, it looks Ok and compatible to Pycom's MicroPython.

robert-hh

@tttadam 1K is too low. The usual recommendation is 4.7 k. But that should not change the figure. The question is, whether i2c.scan() returns [64] also in the context of your main code.
If yes, then the port & wires & I2C protocol are OK, and we have to look at the SI7021 module.

tttadam

@robert-hh Okay, I will get bigger resistors, and replace the sensor.
Thank you for the help.

robert-hh

@tttadam I searched a little bit for a module that looks like yours. I found one, and in the backside pictures I see some resistors, which most likely are 10k pull-up resistors, and a three terminal device, which is according to the print-out a LM6206N3 3.3V voltage regulator. The pull-up resistors seem to be connected to 5V. Being 10k, that should not hurt the pycom device, and you do not need external pull-up resistors. And the 5V were OK for your device.

Edit: I see also a dual level shifter, which is intended to connect a 5V side at the micro to the 3.3 V side of the sensor. It is a single FET transistor level shifter.

tttadam

@robert-hh Well I just ran the code again(from 5V without resistors), but I got OS error for every baud rate from 1 to 10000. So I think something most be wrong with this sensor.

Eric73

@tttadam Single transistor level shifter ? A single transistor is an inverter gate, your SDA and SCL signal will be inverted. Please note that i2c is an opendrain bus with pull up. Your SI7021 can be powered by +5V (if its identical to the one showed by robert-hh) but have you checked is your board have 5V-3.3V regulator and have you checked what is SDA and SCL voltage level when you just power SI7021? In SI7021 datasheet absolute maximum rating for SDA and SCL is 3.3V, i have doubts that a SI7021 board have +5V pull-up on SDA and SCL....without power any of the two board, please check your wire continuity with multimeter if you can.

robert-hh

@eric73 said in SI7021 I2C pin setup:

Single transistor level shifter ?

Yes. there is a standard circuit doing this with single MosFET transistor, used by all cheap level shifter. Principle below. LV is low voltage, HV is high voltage, Lx and Hx are the low & high voltage logic. And you see the pull-up resistors on both sides:

P.S.: You could make a picture of the back side of the sensor, just to clarify @Eric73 's question.

nespressif

Hello everyone, I have tested with an HTU21d sensor that is very similar to the SI7021 and with the SI7021 class I also get the "I2C bus error". However with the code in the main.py, it works perfectly, and without external resistances, I do not understand why I get the error with the class, if everything seems to be fine.
I use this one:

from machine import I2C
import time

i2c = I2C(0, I2C.MASTER)
print(i2c.scan()) #I get [64] is 0x40

i2c.writeto(0x40, 0xf3) #read temp no hold
time.sleep_ms(25) 
temp = i2c.readfrom(0x40, 3)
temp2 = temp[0] << 8
temp2 = temp2 | temp[1]
print((175.72 * temp2 / 65536) - 46.85)

I hope it helps you, Regards

robert-hh

@nespressif The difference is in writing the command. In your example, you are writing a single int value,
i2c.writeto(0x40, 0xf3) #read temp no hold
wheras in the class it is two bytes, where the first byte contains the command, and the second a zero.

  def write_command(self, command_byte):
    self.cbuffer[0] = command_byte
    self.i2c.writeto(self.addr, self.cbuffer)

Since you have a suitable device at hand, could you try to change the class accordingly. e.g. like below:

  def readTemp(self):
    self.i2c.writeto(self.addr, CMD_MEASURE_TEMPERATURE)
    sleep_ms(25)
    temp = self.i2c.readfrom(self.addr,3)
    temp2 = temp[0] << 8
    temp2 = temp2 | temp[1]
    return (175.72 * temp2 / 65536) - 46.85

nespressif

@robert-hh I'm sorry, but I don't understand what you want me to do. I've already tried creating a SI7021(i2c) class object and I get that error. Maybe that's how it works, I'll try it:

def write_command(self, command_byte):
    #self.cbuffer[0] = command_byte
    self.i2c.writeto(self.addr, command_byte)

robert-hh

This post is deleted!

robert-hh

@nespressif @tttadam I just verified that change to write_command(), and it works.
Setting: Vin of the SI7021 module at 3.3V, not external pull-up resistors.

tttadam

@robert-hh 1sorry for the delay.
I took pictures of the sensors.
https://photos.app.goo.gl/d1p1NSwZaXU41QbK8

tttadam

@robert-hh Sorry But it is not clear for me. What sould I change in SI7021.py file?
Thnak you

robert-hh

@tttadam The board looks fine. Not additional resistors required. In the SI7021 class there is a function call write_command. Change that function into:

def write_command(self, command_byte):
    self.i2c.writeto(self.addr, command_byte)

tttadam

@robert-hh well I ran my little baudrate finder again with modflyed write_command method.
But I got tha same result, exept it frozed at 2964 baud.

I am not sure what am I missing.

robert-hh

@tttadam Don't care about the baud rate. Here is works well at the default baud rate (I purchased I device for testing). This is the class code which works:

from time import sleep_ms
from machine import Pin, I2C

# Default Address
SI7021_I2C_DEFAULT_ADDR = 0x40

# Commands
CMD_MEASURE_RELATIVE_HUMIDITY_HOLD_MASTER_MODE = 0xE5
CMD_MEASURE_RELATIVE_HUMIDITY = 0xF5
CMD_MEASURE_TEMPERATURE_HOLD_MASTER_MODE = 0xE3
CMD_MEASURE_TEMPERATURE = 0xF3
CMD_READ_TEMPERATURE_VALUE_FROM_PREVIOUS_RH_MEASUREMENT = 0xE0
CMD_RESET = 0xFE
CMD_WRITE_RH_T_USER_REGISTER_1 = 0xE6
CMD_READ_RH_T_USER_REGISTER_1 = 0xE7
CMD_WRITE_HEATER_CONTROL_REGISTER = 0x51
CMD_READ_HEATER_CONTROL_REGISTER = 0x11

class SI7021(object):
  def __init__(self, i2c=None):
    self.i2c = i2c
    self.addr = SI7021_I2C_DEFAULT_ADDR
    self.cbuffer = bytearray(2)
    self.cbuffer[1] = 0x00
    
  def write_command(self, command_byte):
    self.i2c.writeto(self.addr, command_byte)

  def readTemp(self):
    self.write_command(CMD_MEASURE_TEMPERATURE)
    sleep_ms(25)
    temp = self.i2c.readfrom(self.addr,3)
    temp2 = temp[0] << 8
    temp2 = temp2 | temp[1]
    return (175.72 * temp2 / 65536) - 46.85

  def readRH(self):
    self.write_command(CMD_MEASURE_RELATIVE_HUMIDITY)
    sleep_ms(25)
    rh = self.i2c.readfrom(self.addr, 3)
    rh2 = rh[0] << 8
    rh2 = rh2 | rh[1]
    return (125 * rh2 / 65536) - 6

It is called with:

from machine import I2C
i2c=I2C(0, I2C.MASTER)

from SI7021 import SI7021
m = SI7021(i2c)
print(m.readRH(), m.readTemp())

Device connected at P9 = SDA and P10 = SCL, Vin at 3.3V

tttadam

@robert-hh

Traceback (most recent call last):
  File "main.py", line 6, in <module>
  File "/flash/lib/SI7021.py", line 32, in readTemp
OSError: I2C bus error

still the same. If it's working on your side, than my sensor must be dead.

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SI7021 I2C pin setup (SOLVED)

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