I2C [2] - why does IIC need to use open drain output and pull-up resistor bug

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Project scenario ：

Recently in tune ICM20602 modular ( A six axis gyroscope and accelerometer ), Use IIC Communication protocol , During this process, I encountered a problem that bothered me for a long time .

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Problem description

IIC Correct agreement , But the reading fails all the time .

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Cause analysis ：

Finally, it was found that... Was not configured GPIO Open drain output

Push pull output and open drain output

Push pull output : Output logic 0, be N-MOS Activate ; Output logic 1, be P-MOS Activate .

Open drain output : When the pull-up resistor is not connected , Output logic 0, be N-MOS Activate ; Output logic 1,P-MOS Will not activate , No high level output . When connecting the pull-up resistor , Output logic 0, be N-MOS Activate ; Output logic 1,P-MOS Activate , Can output high level .

That is, the open drain output if Do not connect the pull-up resistor , No, Output high level Ability . If the open drain output is required to have the ability to output high level, a pull-up resistor shall be connected . At present, many single-chip computers GPIO The pull-up resistor can be configured by software .

The figure on the left shows the open drain output ( Connect the pull-up resistor ), The picture on the right shows push-pull output

The function of open drain output

Prevent short circuit : In some cases ( For example, the bus ), Multiple GPIO Ports may be connected on the same line , There is a certain GPIO Output high level , the other one GPIO Output low level . If push-pull output is used , You'll find this GPIO Of VCC And another one. GPIO Of GND Connected together , It's a short circuit ( It's cool ). What if you change to open drain output ? VCC and GND An extra resistor , So the circuit is safe . Therefore, the bus generally uses open drain output .

Line and : Open drain output can also be realized Line and ( To baidu ), Reduce an and gate , Simplify the circuit .

IIC Why use open drain output and pull-up resistor

1. IIC The protocol supports multiple master devices and multiple slave devices on the same bus , If open drain output is not used , And push-pull output , There will be a short circuit between the main equipment .
2. As for why we need to pull up the resistor , That's because IIC Communication requires the ability to output high levels .

In order to realize the arbitration when multiple master devices preempt the bus .IIC There are only two wires (SCL and SDA), How to determine which master device occupies the bus ( First come first served, of course ).

Suppose the main equipment A Need to start the IIC, He needs to be in SCL High voltage usually , take SDA Convert from high level to low level as start signal . Main equipment A In the SDA After pulling up , It needs to be checked again SDA The level of .

Why? ? Because the line and . If the main equipment A pull up SDA when , There are already other main devices that will SDA Pull down . because 1 & 0 = 0 Then the main equipment A Checking SDA Peacetime , You'll find that it's not high , It's a low level . It indicates that other master devices preempt the bus earlier than it , Main equipment A Only the occupied bus can be abandoned . If it's high , Can occupy .

This is the open drain output at IIC Another role in communication .

SDA Is a high level , Describe the main equipment A It can occupy the bus , Then the main equipment A take SDA Pull it down , Start communicating .SDA Is a low level , It means someone has got there first , Main equipment A The bus cannot be occupied , The end of the communication .

therefore , simulation IIC Be sure to will GPIO The port is set to open drain output and a pull-up resistor is added .( Hardware IIC It will be automatically configured as open drain output ).

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Solution ：

Finally, let's talk about why we used Push pull output IIC Read RTC No problem , This is because the pull-up resistance is different ,RTC The pull-up resistor can normally pull up and pull down the low level even if the push-pull output . According to the above , You can check MCU Of datasheet, confirm IO Of PMOS and NMOS The impedance of , Calculate the voltage .

There is another simple and crude way , Problems can also be found by directly using the oscilloscope to look at the waveform