I2C clocks stop after slave ack the address

tommino — Wed, 16 Apr 2025 06:45:59 GMT

Hello community,

I am using the LPC824 I2C to talk with a IC slave. The goal is to have this kind of communication as reported in the slave manual

Precondition: I2C HW is correctly working in my board since I can talk to different I2C slaves without issues ( I mean different type of devices...i have more than one unit of the I2C slave that is giving me the problem and I tried all of them and nothing changes).

The problem is that everything is fine until the part pointed by the arrow: LPC824 sends out the slave address which is ACK by the slave, then I have the register and then the I see another ACK in the SDA line but from now on: SCL line is stopped and no clock is present, SDA line keeps staying low forever. I think that the problem comes from the fact that no clock is present ( see oscilloscope screenschot attached).

The code is the following:

if((LPC_I2C0->STAT & MSTSTATE) == MSTSTATE_IDLE){ /* if idle */
LPC_I2C0->MSTDAT=(devaddr<<1); /* send addr w/ R/W bit =0 */
LPC_I2C0->MSTCTL=(1<<MSTSTART); /* send start condition */
}

while(!(LPC_I2C0->STAT & MSTPENDING)) /* wait for i2c0 idle or pending */

// send register

LPC_I2C0->MSTDAT=(reg_address);

LPC_I2C0->MSTCTL=(1<<MSTCONTINUE);

while(!(LPC_I2C0->STAT & MSTPENDING)); /* wait for i2c0 idle or pending */

LPC_I2C0->MSTDAT=(devaddr<< 1)|0x1; /* send addr w/ R/W bit =1 */
LPC_I2C0->MSTCTL=(1<<MSTCONTINUE);

while(!(LPC_I2C0->STAT & MSTPENDING)); /* wait for i2c0 idle or pending */
value1=LPC_I2C0->MSTDAT;
LPC_I2C0->MSTCTL=(1<<MSTCONTINUE);

LPC_I2C0->MSTCTL=(1<<MSTSTOP)

Any idea where is the bug?

Re: I2C clocks stop after slave ack the address

tommino — Wed, 16 Apr 2025 07:38:24 GMT

I forgot the scope screenshot

Re: I2C clocks stop after slave ack the address

Alice_Yang — Fri, 18 Apr 2025 08:57:28 GMT

Hello @tommino

Thank you for your contacting us.

In an I2C read operation, when switching from "writing the register address" to "reading data", a Repeated Start signal needs to be sent instead of directly sending the read address.

The current code uses MSTCONTINUE before sending the read address. This causes the timing to not comply with the standard I2C specification. Therefore, please change LPC_I2C0->MSTCTL=(1<<MSTCONTINUE); to LPC_I2C0->MSTCTL = (1 << MSTSTART); before the read operation.

Alice

LPC MicrocontrollersのトピックI2C clocks stop after slave ack the address

I2C clocks stop after slave ack the address

Re: I2C clocks stop after slave ack the address

Re: I2C clocks stop after slave ack the address