I2C pull-up

[KentOverton]

I'm trying to communicate with a ST MicroElectronics ST25R3918 NFC transceiver via I2C. 

I'm using a nrf52dk_nrf52832, NRF SDK 2.4.0 in vscode.

I want to use only external pull-up resistors on SDA and SCL (the ST chip  wants to run a 1MHz).

app.overlay has 

&i2c1 {

    status = "okay";

    pinctrl-0 = <&i2c1_default>;

    pinctrl-1 = <&i2c1_sleep>;

   pinctrl-names = "default", "sleep";

};

with pinctrl of 

i2c1_default: i2c1_default {

        group1 {

            psels = <NRF_PSEL(TWIM_SDA, 0, 30)>,

                <NRF_PSEL(TWIM_SCL, 0,  29)>;

                   

        };

    };

    i2c1_sleep: i2c1_sleep {

        group1 {

            psels = <NRF_PSEL(TWIM_SDA, 0, 30)>,

                <NRF_PSEL(TWIM_SCL, 0, 29)>;

            low-power-enable;

        };

    };

I believe that the internal pull-ups (13.5K) are enabled, but I can't find the appropriate pinctrl statement to disable them.

Can you please point me in the right direction?

[Susheel Nuguru]

You need to add pin bias as mentioned in this documentation for the pinctrl. If you want no pull-ups then you need to disable bias like below

&pinctrl {
	i2c1_default: i2c1_default {
		group1 {
			psels = <NRF_PSEL(TWIM_SDA, 0, 30)>,
				<NRF_PSEL(TWIM_SCL, 0, 29)>;
			bias-disable;
		};
	};

	i2c1_sleep: i2c1_sleep {
		group1 {
			psels = <NRF_PSEL(TWIM_SDA, 0, 30)>,
				<NRF_PSEL(TWIM_SCL, 0, 29)>;
			bias-disable;
			low-power-enable;
		};
	};
};

&i2c0 {
	status = "okay";
	pinctrl-0 = <&i2c1_defaulti2c1_default>;
	pinctrl-1 = <&i2c1_sleep>;
	pinctrl-names = "default", "sleep";

	wm8731: wm8731@1a {
		compatible = "wolfson,wm8731";
		reg = <0x1a>;
	};
};

See here that I have added bias-disable to make sure that the pinctrl knows not to enable any pulls on the pins

[KentOverton]

When I add bias-disable; to app.overlay VSCODE complains  with "Property not mentioned in "nordic,nrf-pinctrl:child:child""

Should I worry about this?

[Susheel Nuguru]

Hmm, I need to ask my colleague about this. It might be possible that the child node needs to support the default pin bias in the device tree. I look into this and come back to you with more info.

[Naeem Maroof]

Hello Martin,

We can see that these properties are defined in nordic,nrf-pinctrl

If some property that is not there and is mentioned in the overlay then it would generate the error and the project would not compile.

I recommend you to compile the project and check the output zephyr dts (./build/zephyr/zephyr.dts) and you should see the properties added / modified via the overlay over there.

I also recommend you to check the respective registers. For example, in the below example, I am disabling the pull up/down for the I2C pins (P0.29 and P0.30).

After compiling and flashing the code, I am reading the respective registers to see that the properties set in the overlay are reflected. I am reading the TWIM0 registers:

reg 0x500: Twi is Enabled / Disabled (value 0x6 means Enabled)
reg 0x508 and 50C: Pins configured (0x1D = 29, 0x1E=30)

And then I am reading the GPIO registers for specific pins

reg 0x71C and 0x720: for the pin#7 and 8, the value 0xC = b'1100 = PULL-UP

reg 0x774 and 0x778: for the pin#29 and 30, the value is 0x600 (Pull Disabled)

With regards,

Naeem

[KentOverton]

Naeem,

I tried several things to address this.

I noticed that even when VSCode complains about 'bias-disable' (or for that matter 'bias-pull-up' ) my project still builds. But if I put 'bias-pull-x' in the pinctrl the build fails.

I built the project three times. Once with no bias statement in pinctrl, once with bias-disable in pinctrl and once with bias-pull-up in pinctrl.

After each build I flashed my board and listed the registers.

Note that I am using I2C1 - my program uses SPI0.

With no bias statement in pinctrl I get

C:\Users\kover>nrfjprog --memrd 0x40004500
0x40004500: 00000000 |....|
C:\Users\kover>nrfjprog --memrd 0x40004508
0x40004508: 0000001D |....|
C:\Users\kover>nrfjprog --memrd 0x4000450c
0x4000450C: 0000001E |....|
C:\Users\kover>nrfjprog --memrd 0x40004524
0x40004524: 06680000 |..h.|
C:\Users\kover>nrfjprog --memrd 0x50000774
0x50000774: 0000060D |....|
C:\Users\kover>nrfjprog --memrd 0x50000778
0x50000778: 0000060D |....|

With a 'bias-disable' I get

C:\Users\kover>nrfjprog --memrd 0x40004500
0x40004500: 00000000 |....|
C:\Users\kover>nrfjprog --memrd 0x40004508
0x40004508: 0000001D |....|
C:\Users\kover>nrfjprog --memrd 0x4000450c
0x4000450C: 0000001E |....|
C:\Users\kover>nrfjprog --memrd 0x40004524
0x40004524: 06680000 |..h.|
C:\Users\kover>nrfjprog --memrd 0x50000774
0x50000774: 0000060D |....|
C:\Users\kover>nrfjprog --memrd 0x50000778
0x50000778: 0000060D |....|

With a 'bias-pull-up' I get

C:\Users\kover>nrfjprog --memrd 0x40004500
0x40004500: 00000000 |....|
C:\Users\kover>nrfjprog --memrd 0x40004508
0x40004508: 0000001D |....|
C:\Users\kover>nrfjprog --memrd 0x4000450c
0x4000450C: 0000001E |....|
C:\Users\kover>nrfjprog --memrd 0x40004524
0x40004524: 06680000 |..h.|
C:\Users\kover>nrfjprog --memrd 0x50000774
0x50000774: 0000060C |....|
C:\Users\kover>nrfjprog --memrd 0x50000778
0x50000778: 0000060C |....|

My results don't match yours. at 0x00004500, 0x40004524, ox50000774and 0x50000778  

The differences at 0x40004524 are probably because I'm doing a err=i2c_configure(i2c1,I2C_SPEED_SET(I2C_SPEED_FAST));
The value however does not match what I see in the nRF52832 Product Specification.for 400kHZ.

The values at 0x50000774 and 0X50000778 are the most confusing.
They are the same for compiling with a bias statement and with a 'bias-disable'.
This seems consistent with the Zephyr documentation which says
"bias-disable: Disable pull-up/down (default behavior, not required)."

And changes when I use 'bias-pull-up'.

But I don't understand the values at all.

[Naeem Maroof]

Hello

Is there any progress on this?

KentOverton said:

C:\Users\kover>nrfjprog --memrd 0x40004500
0x40004500: 00000000 |....|

This means the device is not enabled.