Fluctuating Idle Current when Using SPI1 (nRF52832)

[Crisvin_K]

Hello, 

I am using the ANNA-B112 module with nRF52832 to communicate to an AD5940 AFE using SPI1. I am also communicating with three other sensors using I2C0. This project uses nRF Connect SDK v3.0.0. I have posted the .dts and .overlay files that I am using for this project below. 

ubx_evkannab1_nrf52832.dts

/*
 * Copyright (c) 2021 u-blox AG
 *
 * SPDX-License-Identifier: Apache-2.0
 */

/dts-v1/;
#include <nordic/nrf52832_qfaa.dtsi>
#include "ubx_evkannab1_nrf52832-pinctrl.dtsi"
#include <zephyr/dt-bindings/input/input-event-codes.h>

/ {
	model = "u-blox EVK-ANNA-B1 NRF52832";
	compatible = "u-blox,ubx_evkannab1_nrf52832";

	chosen {
		zephyr,console = &uart0;
		zephyr,shell-uart = &uart0;
		zephyr,uart-mcumgr = &uart0;
		zephyr,bt-mon-uart = &uart0;
		zephyr,bt-c2h-uart = &uart0;
		zephyr,sram = &sram0;
		zephyr,flash = &flash0;
		zephyr,code-partition = &slot0_partition;
	};

	leds {
		compatible = "gpio-leds";
		led0: led_0 {
			/* ANNA-B1 GPIO_29 */
			gpios = <&gpio0 27 GPIO_ACTIVE_LOW>;
			label = "Red LED";
		};
		led1: led_1 {
			/* ANNA-B1 GPIO_30 */
			gpios = <&gpio0 25 GPIO_ACTIVE_LOW>;
			label = "Green LED";
		};
		led2: led_2 {
			/*ANNA-B1 GPIO_31 */
			gpios = <&gpio0 26 GPIO_ACTIVE_LOW>;
			label = "Blue LED";
		};
	};

	pwmleds {
		compatible = "pwm-leds";
		red_pwm_led: red_pwm_led {
			pwms = <&pwm0 0 PWM_MSEC(20) PWM_POLARITY_INVERTED>;
		};
		green_pwm_led: green_pwm_led {
			pwms = <&pwm0 1 PWM_MSEC(20) PWM_POLARITY_INVERTED>;
		};
		blue_pwm_led: blue_pwm_led {
			pwms = <&pwm0 2 PWM_MSEC(20) PWM_POLARITY_INVERTED>;
		};
	};

	buttons {
		compatible = "gpio-keys";
		button0: button_0 {
			/*  EVK-ANNA-B1 SW1 button */
			gpios = <&gpio0 25 (GPIO_PULL_UP | GPIO_ACTIVE_LOW)>;
			label = "Push button switch SW1";
			zephyr,code = <INPUT_KEY_0>;
		};
		button1: button_1 {
			/* EVK-ANNA-B1 SW2 button */
			gpios = <&gpio0 24 (GPIO_PULL_UP | GPIO_ACTIVE_LOW)>;
			label = "Push button switch SW2";
			zephyr,code = <INPUT_KEY_1>;
		};
	};

	arduino_header: connector {
		compatible = "arduino-header-r3";
		#gpio-cells = <2>;
		gpio-map-mask = <0xffffffff 0xffffffc0>;
		gpio-map-pass-thru = <0 0x3f>;
		gpio-map = <0 0 &gpio0 4 0>,	/* A0 */
			   <1 0 &gpio0 5 0>,	/* A1 */
			   <2 0 &gpio0 28 0>,	/* A2 */
			   <3 0 &gpio0 29 0>,	/* A3 */
			   <4 0 &gpio0 30 0>,	/* A4 */
			   <5 0 &gpio0 31 0>,	/* A5 */
			   <6 0 &gpio0 2 0>,	/* D0 */
			   <7 0 &gpio0 3 0>,	/* D1 */
			   <8 0 &gpio0 19 0>,	/* D2 */
			   <9 0 &gpio0 11 0>,	/* D3 */
			   <10 0 &gpio0 27 0>,	/* D4 */
			   <11 0 &gpio0 26 0>,	/* D5 */
			   <12 0 &gpio0 10 0>,	/* D6 */
			   <13 0 &gpio0 9 0>,	/* D7 */
			   <14 0 &gpio0 14 0>,	/* D8 */
			   <15 0 &gpio0 24 0>,	/* D9 */
			   <16 0 &gpio0 22 0>,	/* D10 */
			   <17 0 &gpio0 23 0>,	/* D11 */
			   <18 0 &gpio0 18 0>,	/* D12 */
			   <19 0 &gpio0 20 0>,	/* D13 */
			   <20 0 &gpio0 15 0>,	/* D14 */
			   <21 0 &gpio0 16 0>;	/* D15 */
	};

	arduino_adc: analog-connector {
		compatible = "arduino,uno-adc";
		#io-channel-cells = <1>;
		io-channel-map = <0 &adc 1>,	/* A0 = P0.4 = AIN1 */
				 <1 &adc 2>,	/* A1 = P0.5 = AIN2 */
				 <2 &adc 4>,	/* A2 = P0.28 = AIN4 */
				 <3 &adc 5>,	/* A3 = P0.29 = AIN5 */
				 <4 &adc 6>,	/* A4 = P0.30 = AIN6 */
				 <5 &adc 7>;	/* A5 = P0.31 = AIN7 */
	};

	/* These aliases are provided for compatibility with samples */
	aliases {
		led0 = &led0;
		led1 = &led1;
		led2 = &led2;
		pwm-led0 = &red_pwm_led;
		pwm-led1 = &green_pwm_led;
		pwm-led2 = &blue_pwm_led;
		red-pwm-led = &red_pwm_led;
		green-pwm-led = &green_pwm_led;
		blue-pwm-led = &blue_pwm_led;
		sw0 = &button0;
		sw1 = &button1;
		watchdog0 = &wdt0;
	};
};

&reg {
	regulator-initial-mode = <NRF5X_REG_MODE_DCDC>;
};

&adc {
	status = "okay";
};

&uicr {
	gpio-as-nreset;
};

&gpiote {
	status = "okay";
};

&gpio0 {
	status = "okay";
};


&uart0 {
	compatible = "nordic,nrf-uarte";
	status = "okay";
	current-speed = <115200>;
	pinctrl-0 = <&uart0_default>;
	pinctrl-1 = <&uart0_sleep>;
	pinctrl-names = "default", "sleep";
};

arduino_i2c: &i2c0 {
	compatible = "nordic,nrf-twi";
	status = "okay";
	pinctrl-0 = <&i2c0_default>;
	pinctrl-1 = <&i2c0_sleep>;
	pinctrl-names = "default", "sleep";
};

&i2c1 {
	compatible = "nordic,nrf-twi";
	/* Cannot be used together with spi1. */
	/* status = "okay"; */
	pinctrl-0 = <&i2c1_default>;
	pinctrl-1 = <&i2c1_sleep>;
	pinctrl-names = "default", "sleep";
};

&pwm0 {
	status = "okay";
	pinctrl-0 = <&pwm0_default>;
	pinctrl-1 = <&pwm0_sleep>;
	pinctrl-names = "default", "sleep";
};

arduino_spi: &spi0 {
	compatible = "nordic,nrf-spi";
	/* Cannot be use simultaneously with i2c0 */
	/* status = "okay"; */
	cs-gpios = <&arduino_header 16 GPIO_ACTIVE_LOW>; /* D10 */
	pinctrl-0 = <&spi0_default>;
	pinctrl-1 = <&spi0_sleep>;
	pinctrl-names = "default", "sleep";
};

&spi1 {
	compatible = "nordic,nrf-spi";
	status = "okay";
	pinctrl-0 = <&spi1_default>;
	pinctrl-1 = <&spi1_sleep>;
	pinctrl-names = "default", "sleep";
};

&flash0 {
	partitions {
		compatible = "fixed-partitions";
		#address-cells = <1>;
		#size-cells = <1>;

		boot_partition: partition@0 {
			label = "mcuboot";
			reg = <0x00000000 0xc000>;
		};
		slot0_partition: partition@c000 {
			label = "image-0";
			reg = <0x0000C000 0x32000>;
		};
		slot1_partition: partition@3e000 {
			label = "image-1";
			reg = <0x0003E000 0x32000>;
		};
		scratch_partition: partition@70000 {
			label = "image-scratch";
			reg = <0x00070000 0xa000>;
		};
		storage_partition: partition@7a000 {
			label = "storage";
			reg = <0x0007a000 0x00006000>;
		};
	};
};

ubx_evkannab1_nrf52832.overlay

&i2c1 {status = "disabled";};
&i2s0 {status = "disabled";};
&uart0 {status = "disabled";};
&arduino_i2c {status = "disabled";};
&adc {status = "disabled";};
&pwm0 {status = "disabled";};
arduino_spi: &spi0 {status = "disabled";};
	

/ {
    gpiocustom {
        compatible = "gpio-keys";

		// BQ25155 LP: nRF Pin 3 -> ANNA-B112 Pin 19
        gpiocustom0: gpiocustom_0 {
            label = "GPIO_BQ25155LP";
            gpios = <&gpio0 3 (GPIO_OPEN_SOURCE)>;
        };

		// AD5940 CS: nRF Pin 2 -> ANNA-B112 Pin 20
		gpiocustom1: gpiocustom_1 {
			label = "GPIO_AD5940CS";
			gpios = <&gpio0 2 (GPIO_PULL_UP | GPIO_ACTIVE_LOW)>;
		};

		// AD5940 RESET: nRF Pin 31 -> ANNA-B112 Pin 25
		gpiocustom2: gpiocustom_2 {
			label = "GPIO_AD5940RESET";
			gpios = <&gpio0 31 (GPIO_PULL_UP | GPIO_ACTIVE_LOW)>;
		};

		// AD5940 INT0: nRF Pin 4 -> ANNA-B112 Pin 24
		gpiocustom3: gpiocustom_3 {
			label = "GPIO_AD5940INT0";
			gpios = <&gpio0 4 (GPIO_PULL_DOWN | GPIO_ACTIVE_LOW)>; 
		};
    };

    aliases {
        bq25155lp = &gpiocustom0;
		ad5940cs = &gpiocustom1;
		ad5940reset = &gpiocustom2;
		ad5940int0 = &gpiocustom3;
    };
};

&gpio0 {
	status = "okay";
	sense-edge-mask = < 0xffffffff >;
};

&spi1 {
    compatible = "nordic,nrf-spi";
    status = "okay";
    pinctrl-0 = <&spi1_default>;
    pinctrl-1 = <&spi1_sleep>;
    pinctrl-names = "default", "sleep";
    
	// AD5940: Potentiostat
    ad5940: ad5940@0 {
        compatible = "vnd,spi-device";
        reg = <0>;
        spi-max-frequency = <1600000>;
        label = "ad5940";
    };
};

&i2c0 {
    status = "okay";
	pinctrl-0 = <&i2c0_default>;
	pinctrl-1 = <&i2c0_sleep>;
	pinctrl-names = "default", "sleep";

	// BQ25155: Battery Management System
    bq25155: bq25155@6B {
		compatible = "i2c-device";
		status = "okay";
		reg = < 0x6B >;
	};

	// HDC2022: Temperature and Humidity Sensor
	hdc20xx: hdc20xx@40 {
		compatible = "i2c-device";
		status = "okay";
		reg = < 0x40 >;
	};

	// BMA400: Accelerometer
	bma400: bma400@14 {
		compatible = "i2c-device";
		status = "okay";
		reg = < 0x14 >;
	};
};

&pinctrl {
	// I2C Pin Definition
	// SCL: nRF Pin 16 -> ANNA-B112 Pin 15
	// SDA: nRF Pin 15 -> ANNA-B112 Pin 14
    /omit-if-no-ref/ i2c0_default: i2c0_default {
		group1  {
			psels = <NRF_PSEL(TWIM_SCL, 0, 16)>,
					<NRF_PSEL(TWIM_SDA, 0, 15)>;
		};
	};

	/omit-if-no-ref/ i2c0_sleep: i2c0_sleep {
		group1  {
			psels = <NRF_PSEL(TWIM_SCL, 0, 16)>,
					<NRF_PSEL(TWIM_SDA, 0, 15)>;
			low-power-enable;
		};
	};

	// SPI Pin Definitions
	// SCK: nRF Pin 20 -> ANNA-B112 Pin 45
	// MOSI: nRF Pin 19 -> ANNA-B112 Pin 35
	// MISO: nRF Pin 24 -> ANNA-B112 Pin 38
	spi1_default: spi1_default {
		group1 {
			psels = <NRF_PSEL(SPIM_SCK, 0, 20)>,
					<NRF_PSEL(SPIM_MOSI, 0, 19)>,
					<NRF_PSEL(SPIM_MISO, 0, 24)>;
		};
	};
	spi1_sleep: spi1_sleep {
		group1 {
			psels = <NRF_PSEL(SPIM_SCK, 0, 20)>,
					<NRF_PSEL(SPIM_MOSI, 0, 19)>,
					<NRF_PSEL(SPIM_MISO, 0, 24)>;
			low-power-enable;
		};
	};
};

/delete-node/ &slot1_partition;
/delete-node/ &scratch_partition;
/delete-node/ &storage_partition;

&flash0 {
	partitions {
		compatible = "fixed-partitions";
		#address-cells = <1>;
		#size-cells = <1>;

		// Boot Partition: 49152 B
		boot_partition: partition@0 {
			label = "mcuboot";
			reg = <0x00000000 0xc000>;
		};
		// Slot0 Partition: 204800 B
		slot0_partition: partition@c000 {
			label = "image-0";
			reg = <0x0000C000 0x32000>;
		};
		// Slot1 Partition: 4096 B
		slot1_partition: partition@3e000 {
			label = "image-1";
			reg = <0x0003E000 0x1000>;
		};
		// Scratch Partition: 4096 B
		scratch_partition: partition@3f000 {
			label = "image-scratch";
			reg = <0x0003F000 0x1000>;
		};
		// Storage Partition (For Encryption Keys): 8192 B
		storage_partition: partition@40000 {
			label = "storage";
			reg = <0x00040000 0x0002000>;
		};
		// File System Partition: 253952 B
		fs_partition: partition@42000 {
			label = "fs";
			reg = <0x42000 0x3e000>;
		};
	};
};

What I am noticing is that initially the idle current of the system will be low as expected around 4-5 uA. However, after a few seconds, the idle current will slowly start to increase and peak at around 500 uA. After some more time the idle current may continue to fluctuate between 4 uA - 500 uA seemingly at random. Functionality of the communication is not affected. 

This behavior is observed on both the ANNA-B112 EVK and on a custom PCB board. On the ANNA-B112 EVK, if I touch the button SW2 with my finger (touch the button housing not press the button down) the idle current will very quickly go back down to 4-5 uA as expected and communication functionality is not affected. 

(Note ANNA-B112 module IO_38 = nRF52832 P0.24)

I believe the issue is related to SPI and GPIOs communicating with the AD5940. If you notice, I use nRF52832 pin 24 for MISO and the ANNA-B112 EVK uses the same pin for SW2. I tried deleting the buttons definition on the .dts file, but this did not change the idle current fluctuation behavior. 

I am not sure this is an EVK related issue because I am also noticing the same behavior on a custom PCB which has no button. For reference I don't use any external pullup or pulldown resistors on SPI, but I do use a 10K pullup for I2C.

What could cause the idle current to fluctuate like this when using SPI?

Thank you, 

Crisvin Kadambathil

[Menon]

Hello,

Which SDK version are you using?

To help narrow down the root cause, please try temporarily disabling the sense-edge-mask in your overlay file. You can either comment out the line or set the mask to 0. This will make the GPIO driver fall back to the default GPIOTE-based interrupt handling instead of the PORT/SENSE workaround.

There is a known anomaly when using GPIOTE for input interrupts, but the extra current from that is only on the order of a few tens of uA much smaller than the ~500 uA jumps you are seeing. So this test is safe and useful for diagnosis.

After removing the sense-edge-mask, please measure the idle current again. If the 4\5 µA to  500 uA fluctuations disappear, then we can confirm that the SENSE-based workaround is contributing to the issue. If the behavior is unchanged, we can rule it out and look elsewhere.

This step will help us isolate whether the interrupt configuration on GPIO0 is causing the unexpected current spikes.

Kind Regards,

Abhijith

[Crisvin_K]

Hello,

Thank you for your response. However, I think I found a solution that solves the fluctuating current by making sure SCK, MOSI, and MISO are pulled down while not communicating. Here is how I updated my .overlay file. 

// SPI Pin Definitions
	// SCK: nRF Pin 20 -> ANNA-B112 Pin 45
	// MOSI: nRF Pin 19 -> ANNA-B112 Pin 35
	// MISO: nRF Pin 24 -> ANNA-B112 Pin 38
	spi1_default: spi1_default {
		group1 {
			psels = <NRF_PSEL(SPIM_SCK, 0, 20)>,
					<NRF_PSEL(SPIM_MOSI, 0, 19)>,
					<NRF_PSEL(SPIM_MISO, 0, 24)>;
			bias-pull-down;
		};
	};
	spi1_sleep: spi1_sleep {
		group1 {
			psels = <NRF_PSEL(SPIM_SCK, 0, 20)>,
					<NRF_PSEL(SPIM_MOSI, 0, 19)>,
					<NRF_PSEL(SPIM_MISO, 0, 24)>;
			bias-pull-down;
			low-power-enable;
		};
	};

I used the line "bias-pull-down" which ensures that SCK, MOSI, and MISO are not floating. Now the current consumption stays at 4 uA and does not fluctuate when I touch the board or leave it alone for extended periods of time and communication still works correctly. 

Is it expected that these pins should be pulled down to minimize current consumption?

Thank you, 

Crisvin K.

[Menon]

Hello,

Good to hear that.

Crisvin_K said:

Is it expected that these pins should be pulled down to minimize current consumption?

If you ask me whether it's mandatory, I would say no.However, from a hardware perspective, floating GPIO inputs can cause significant leakage current. So adding pull-downs on SCK/MOSI/MISO is a recommended low-power practice

Kind Regards,

Abhijith

[hmolesworth]

All GPIO input pins with slow rise/fall or floating inputs exhibit feedthrough leakage when the input happens to float towards the input voltage threshold; this note is from TI but most manufacturers have similar notes:

scba004e

[Crisvin_K]

Hello hmolesworth,

I appreciate you linking this article.

Thank you!

Crisvin K. 

[Crisvin_K]

Hello, 

Yes, I'll keep this low-power practice in mind for the future.

Thank you, 

Crisvin K.