Max Callbacks? and other GPIO problems - nrf52840-MDK board

Lawrence King

I am developing a piece of code for nrf52840 that uses 24 GPIOs. Conveniently the NRF52840-MDK board brings out exactly 24 GPIOs so I have enough hardware on hand. The application needs 12 inputs and 12 outputs. The inputs are based on a used pushing one of 12 switches, and the output is based on proprietary logic based on the 12 inputs (for simplicity just think of it as a 1:1 operation). After working around Problem 1 (see below) I got all of my code running correctly with a small table of 2 inputs and 2 outputs, the call backs work, the logic works, all is good, then I expanded the table to 12 inputs and 12 outputs. This is where things get weird….


Problem 1) GPIO Group


The NRF52840 has two groups of GPIO ports, controller 0 has 32 GPIOs and controller 1 has fewer. All of the 24 GPIOs that go to pins on the nrf52840-MDK board are on controller 0 (along with the button and the LEDs). I had thought that I could call get_device_binding(SW0_GPIO_CONTROLLER) and get a single handle to all 32 bits on the port. Unfortunately when I start  setting them to either input or output if I try to set pins as outputs (assuming I set one pin as input first) they don’t work (with no error return codes). I ended up getting two device handles, with the second being get_device_binding(LED0_GPIO_CONTROLLER). Once I assigned all of the inputs to the first handle, and the outputs to the second handle I was able to configure and operate all of the 24 pins (with exception to the problems below).


Problem 2) Max Callbacks

Next I added callbacks to all 12 of the inputs. The first 8 callbacks add with no issues, the last 4 return error -19. I couldn’t find a kernel configuration parameter for the max number of callbacks. Is it hard coded to 8 somewhere? Of maybe I need to make a memory region larger, which CONFIG do I need to expand?


Problem 3) GPIOs that don’t.

Now the strangest problem. The gpios at P0.05 andP0.07 don’t work. I can program them as outputs, and they drive low once programmed, but I can’t set them high. All the other gpios work as expected. I thought maybe I had damaged the board, but I took a brand new nfr52840-MDK out of the antistatic bag, loaded the same code and I have the same problem. I suspect there is some vestigial setting somewhere in the device tree that causes these two gpios to behave differently. Any hint where I should look? I generated very simplified code to demonstrate the problem (see below). Gpios 4 6 and 8 work, but 5 and 7 don’t.


Lawrence King

Principal Developer

Connected Transport Market Unit



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*  Irdeto Excavator App


 *           main.c


 *           Lawrence King





#include <zephyr.h>

#include <misc/printk.h>

#include <device.h>

#include <gpio.h>


// These should be defined somewhere in Zephyr, but I couldn't find them hence I took this from the MDK schematic


#define GPIO_00                           0            // P0.00              Button input

#define GPIO_01                           1            // P0.01

#define GPIO_02                           2            // P0.02

#define GPIO_03                           3            // P0.03

#define GPIO_04                           4            // P0.04

#define GPIO_05                           5            // P0.05

#define GPIO_06                           6            // P0.06

#define GPIO_07                           7            // P0.07

#define GPIO_08                           8            // P0.08

#define GPIO_09                           9            // P0.09

#define GPIO_10                           10          // P0.10

#define GPIO_11                           11          // P0.11

#define GPIO_12                           12          // P0.12

#define GPIO_13                           13          // P0.13

#define GPIO_14                           14          // P0.14

#define GPIO_15                           15          // P0.15

#define GPIO_16                           16          // P0.16

#define GPIO_17                           17          // P0.17

#define GPIO_18                           18          // P0.18

#define GPIO_19                           19          // P0.19

#define GPIO_20                           20          // P0.20

#define GPIO_21                           21          // P0.21

#define GPIO_22                           22          // P0.22              Green LED

#define GPIO_23                           23          // P0.23              Red LED

#define GPIO_24                           24          // P0.24              Blue LED

#define GPIO_25                           25          // P0.25

#define GPIO_26                           26          // P0.26

#define GPIO_27                           27          // P0.27

#define GPIO_28                           28          // P0.28

#define GPIO_29                           29          // P0.29

#define GPIO_30                           30          // P0.30

#define GPIO_31                           31          // P0.31

#define NUM_GPIOS      32          // Max for controller P0 on nrf52840


#define RED_LED                          GPIO_23

#define GREEN_LED       GPIO_22

#define BLUE_LED          GPIO_24

#define BUTTON                           GPIO_00



void main(void)


              int         i,j,n;

              u32_t    val;

              struct device                    *in_device, *out_device;



struct    foo {

              int         gpio;

              int         state;

              int         dir;        // 0=input, 1=output

} setup_gpios[] = {

                            // gpio                state     dir

                            {BUTTON,          0,           0},         // the button 00

                            {GPIO_04,          0,           1},         // temp foo

                            {GPIO_05,          0,           1},         //3B

                            {GPIO_06,          0,           1},         //1B

                            {GPIO_07,          0,           1},         //5A

                            {GPIO_08,          0,           1}           //5B


#define SETUP_GPIOS              (sizeof(setup_gpios)/sizeof(setup_gpios[0]))




              printk("SETUP_MODE with %d setup_gpios\n", SETUP_GPIOS);


              in_device  = device_get_binding(SW0_GPIO_CONTROLLER);

              out_device  = device_get_binding(LED0_GPIO_CONTROLLER);


              for (i=0; i<SETUP_GPIOS; i++) {

                            if (setup_gpios[i].dir==0) { // input

                                          gpio_pin_configure(in_device, setup_gpios[i].gpio,

                                                        (GPIO_DIR_IN | GPIO_INT | GPIO_INT_EDGE | GPIO_PUD_PULL_UP | GPIO_INT_DEBOUNCE | GPIO_INT_DOUBLE_EDGE ));

                            } else { // output

                                          gpio_pin_configure(out_device, setup_gpios[i].gpio, GPIO_DIR_OUT | GPIO_DS_DFLT_HIGH);


                            setup_gpios[i].state=5;  // all inputs will print out initially

                            printk("\tinitialized GPIO_%02d.\n",setup_gpios[i].gpio);


              gpio_pin_configure(out_device, GREEN_LED, GPIO_DIR_OUT);



              // scan and look for state changes, and slowly pulse the GPIOs so we can look at hem with a voltmeter

              while (1) {

                            for (i=0; i<SETUP_GPIOS; i++) {

                                          if (setup_gpios[i].dir==0) {


                                                        if (val != setup_gpios[i].state) {

                                                                      // input changed state, print it out

                                                                      printk("input GPIO_%02d=%d\n",setup_gpios[i].gpio, val);





                                          if(i==0) {

                                                        gpio_pin_write(out_device, GREEN_LED, (n++)&1); //toggle to show life

                                                        for (j=1; j<SETUP_GPIOS; j++) {

                                                                      if ((setup_gpios[j].gpio == ((n>>1) & 0x07)) && (setup_gpios[j].dir==1) && (setup_gpios[j].gpio!=0)) {

                                                                                    printk("output GPIO_%02d=%d\n",setup_gpios[j].gpio, !(n&1));

                                                                                    gpio_pin_write(out_device, setup_gpios[j].gpio, !(n&1) ); //toggle slowly




                                          gpio_pin_read(in_device,setup_gpios[0].gpio,&val);              //button

                                          if (val != setup_gpios[0].state) {

                                                        gpio_pin_write(out_device, setup_gpios[4].gpio, val ); // GPIO_07 is supposed to track the button

                                                        setup_gpios[0].state=val;            // remember the button state

                                                        printk("GPIO_07 set to %d\n",val);







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