Kitchen timer

Kitchen timer

Kitchen timer based on 4-digit 8-segment display 3461BS and RP Pico

Front view of the PCB

Back view of the PCB

Electronics

The used display is a common anode display. The datasheet specifies 10 mA typical current for the optical characteristics of a segment. The GPIOs of RP Pico can provide 12 mA max. The built-in 3V3OUT voltage regulator can output up to 300 mA, which is sufficient for all 8 segemts. Not the full 32 segments are needed since the digits are displayed with a given refresh rate sequentially once at a time. Simulation using LTSpice and PNP transistor BCS327-25 leads to the choice of resistor value

s with 220 Ohm and 22k. For the edge cases "1 segment" and "8 segments" the current limitations are fulfilled.

Zephyr RTOS

Setting PWM on a pin

1. Start by enabling PWM in the prj.conf file: CONFIG_PWM=y

2. In the .overlay file, enable the node pwm_led0 by setting the status:

   &pwm_led0 {
   status = "okay";
   };

3. Check the PWM generator corresponding to the GPIO. For RP Pico, these are:

   It results in a table:

   PWM channel Nr.	PWM channel	GPIO
   0	0A	0
   1	0B	1
   2	1A	2
   ..	..	..
   15	7B	15
   0 ❗	0A	16
   1	0B	17

   This means that to set PWM ON GPIO 27, we will need PWM 5B with the number 11.

4. In the .dts file (not the overlay!), edit the pwm_leds node:

    pwm_leds {
    	compatible = "pwm-leds";
    	status = "disabled";
    	pwm_led0: pwm_led_0 {
    		pwms = <&pwm 11 PWM_MSEC(20) PWM_POLARITY_NORMAL>;
    		label = "PWM_LED";
    	};
    };

5. Next, set the PWM on the specific pin. In the rpi_pico-common.dtsi:

    &pwm {
    pinctrl-0 = <&pwm_ch5b_default>;
    pinctrl-names = "default";
    };

6. Finally, add the pinmux in rpi_pico-pinctrl.dtsi for GPIO 27:

   pwm_ch5b_default: pwm_ch5b_default {
    	group1 {
    		pinmux = <PWM_5B_P27>;
    	};
    };

7. Follow the examples such as PWM Blinky and compile them with the changed devicetree. If it works, start coding your own application.

Device tree definitions

Device tree nodes are aliased like in the picture below. The change from - to _ is mandatory!

In the .c code, the alias is used like:

static const struct pwm_dt_spec my_variable = PWM_DT_SPEC_GET(DT_ALIAS(buzzer_dev));

Links

• https://www.electronics-tutorials.ws/blog/7-segment-display-tutorial.html

• https://www.instructables.com/12-Pin-7-Segment-Display-Wiring-Tutorial/

• https://www.reddit.com/r/AskElectronics/comments/oti47u/what_is_the_purpose_of_the_transistors_when_using/

• https://www.youtube.com/watch?v=sKJuzInKj_M

• https://www.youtube.com/watch?v=256VQ6eVNng

• https://www.youtube.com/watch?v=Fa7BHbPAM98

• Switch debouncing https://zephyrproject.org/how-to-debounce-button-inputs-in-a-rtos/

• PWM https://blog.golioth.io/adding-pwm-sound-in-zephyr/