Electronic – I2C – Where’s the ACK

i2cpullup

I'm working on I2C master driver for interfacing a particular peripheral.
The problem I have, is that when the 9th clock arrives, the slave device doesn't pull down the SDA line.

I checked the signals with a scope, and everything looks like fine. the address is fine.
I'm not using external pull-up resistors, I'm using the internal pullup capability of my MCU (EFM32GG).

I'm not an electrical engineer, and my experience is from the software side, so this question may be silly, but is there a chance that the device can't pull down the line because the pull up drive is too high ?

Can there be another explanation for this problem (except for the obvious one, that the slave chip is dead)?

Best Answer

With the scope, you should see some sort of effect on the data line, even if the line is being pulled up too strongly. It may drop only half a volt or something, not enough to be read as a zero.

There's also the possibility of the slave address being wrong. Sometimes the address is specified as 7-bit and sometimes as 8-bit. The latter will be 2x the former. Free advice: Write a loop to try all the addresses and stop if it finds one that responds.

Related Solutions

STM32F207 I2C errors

Sorry for the delay in mentioning this, but I have successfully solved this problem by using the STM32 CPAL library available from ST. I have tested this library with the onboard accelerometer by reading the 'WHO_AM_I' register in the accelerometer. The code for this is:

#include "cpal_i2c.h"

int main()
{
    // Configuration
    CPAL_TransferTypeDef RxStruct;
    uint8_t RxBuf;
    RxStruct.pbBuffer = &RxBuf;
    RxStruct.wAddr1 = 0x39;
    // Initialization
    CPAL_I2C_StructInit(&I2C1_DevStructure);
    I2C1_DevStructure.CPAL_Mode = CPAL_MODE_MASTER;
    I2C1_DevStructure.CPAL_ProgModel = CPAL_PROGMODEL_DMA;
    I2C1_DevStructure.pCPAL_I2C_Struct->I2C_ClockSpeed = 100000;
    I2C1_DevStructure.pCPAL_TransferRx = &RxStruct;
    I2C1_DevStructure.pCPAL_TransferTx = pNULL;
    CPAL_I2C_Init(&I2C1_DevStructure);
    // Communication
    RxStruct.wNumData = 1;
    RxStruct.wAddr2 = 0x0F;
    if(CPAL_I2C_Read(&I2C1_DevStructure) != CPAL_PASS)
    {
        // Error
    }
    while(I2C1_DevStructure.CPAL_State != CPAL_STATE_READY);
    while(1);
    return 0;
}

Electronic – I2C SCL ground issue

Problem:

I powered on board 1 [...] As soon as I plug the I2C connector to board 2 (it's still powered off), SCL and SDA get pulled low.

This is completely expected behaviour. You cannot connect both powered and unpowered devices to the same I²C bus, without additional precautions (or unless this is explicitly stated as being supported by that I²C device). In your case, board 2 may also misbehave afterwards as you are likely to be exceeding the maximum allowed voltage connected to an unpowered MCU pin on board 2.

That is because you are effectively trying to power the MCU on (unpowered) board 2 via its I²C bus pins, which are receiving power through the pull-up resistors on (powered) board 1.

Solution:

Use one of the various existing methods of I²C bus isolation between board 1 and board 2.

Since you say that your setup allows you to connect the Gnd between the two boards, I don't see a requirement for the more expensive and complex galvanic isolation methods.

Instead the simple MOSFET-based circuit, often used as an I²C bus level-shifter from the old Philips (now NXP) AppNote AN97055 can be used. Using the simple circuit in section 2.3 of the datasheet, you connect board 2 to the V_DD1 side of the circuit:

[Although not shown on the above circuit diagram, Gnd (0 V) must also be connected between the V_DD1 and V_DD2 sides.]

Using the above circuit, when the V_DD1 (board 2) side is unpowered, it also receives no power via the I²C bus signals, even when the other V_DD2 (board 1) side of the circuit is powered. Therefore it stops the problem explained above, as described in this excerpt from the Philips AppNote:

Section 2.3.4 from Philips AppNote AN97055

Various vendors sell I²C level-shifter PCBs with this simple circuit using suitable MOSFETs, if you don't have them yourself. However, note that such PCBs often have pull-up resistors fitted. Since you already have pull-ups installed, you must remove any duplicate pull-up resistors from wherever is easiest (probably by removing them from the additional PCB).