I'm trying to get two identical joystics (Saitek cyborg 3D) running with the USB-Host-Shield on a Mega 2560 and an USB-Hub.

I already get the event-messages of both of them:

#include <usbhid.h>
#include <hiduniversal.h>
#include <usbhub.h>

// Satisfy IDE, which only needs to see the include statment in the ino.
#ifdef dobogusinclude
#include <spi4teensy3.h>
#include <SPI.h>

#include "hidjoystickrptparser.h"

USB Usb;
USBHub Hub(&Usb);
HIDUniversal Hid1(&Usb);    // first Joystick
HIDUniversal Hid2(&Usb);    // second Joystick

JoystickEvents Joy1Events;
JoystickEvents Joy2Events;

JoystickReportParser Joy1(&Joy1Events);
JoystickReportParser Joy2(&Joy2Events);

void setup() {
#if !defined(__MIPSEL__)
        while (!Serial); // Wait for serial port to connect - used on Leonardo, Teensy and other boards with built-in USB CDC serial connection

        if (Usb.Init() == -1)
                Serial.println("OSC did not start.");


        if (!Hid1.SetReportParser(0, &Joy1))
                ErrorMessage<uint8_t > (PSTR("SetReportParser1"), 1);
        if (!Hid2.SetReportParser(0, &Joy2))
                ErrorMessage<uint8_t > (PSTR("SetReportParser2"), 1);

void loop() {
        Serial.print("\tX1: ");
        PrintHex<uint8_t > (JoystickEvents::mostRecentEvent.X, 0x80);
        Serial.print("\tY1: ");
        PrintHex<uint8_t > (JoystickEvents::mostRecentEvent.Y, 0x80);
        Serial.print("\tX2: ");
        PrintHex<uint8_t > (JoystickEvents::mostRecentEvent.Z1, 0x80);
        Serial.print("\tY2: ");
        PrintHex<uint8_t > (JoystickEvents::mostRecentEvent.Z2, 0x80);
        Serial.print("\tRz: ");
        PrintHex<uint8_t > (JoystickEvents::mostRecentEvent.Rz, 0x80);


My question is: How can I distinguish the events from the different joysticks.

I'm trying to get some kind of port-information from the USB-Hub. Unfortunately something like


within the JoystickReportParser doesn't do the job as it always returns 0.

Any ideas?

  • look at the hub demo – jsotola Jan 29 '18 at 8:54
  • That's what I'm doing since several days but I don't get the clue. I get message-events but how do I recognize from which port they do come? In the hub demo there is this function: PrintAllAddresses() which takes a pointer to the device. But it's called within this statement: Usb.ForEachUsbDevice(&PrintAllAddresses); – Palmstroem Jan 29 '18 at 9:02
  • In the hub demo there is this function: PrintAllAddresses() which takes a pointer to the device. But it's called within this statement: Usb.ForEachUsbDevice(&PrintAllAddresses); So how would I call something like this from within the JoytickReportParser or the eventhandler OnGamepadChanged ? – Palmstroem Jan 29 '18 at 9:08
  • Or how do I get a pointer to the device from which a message-event comes in? – Palmstroem Jan 29 '18 at 9:09
  • To be honest. I'm not so familiar with all this pointer arithmetic. All these -> :: ! * . Is still a little bit wired to me. – Palmstroem Jan 29 '18 at 9:13

Not quite the question that was asked but part of the problem.

This is a simple introduction to pointers.

int variable = 42;                              // Create a variable
int* pointer = &variable;                       // Create a pointer to the variable (by getting its address)
*pointer = 99;                                  // Set the address to a new value
printf ("variable has the value %d", variable); // Print 99.

&x get the address of x

*y dereferences the pointer y, which means it 'turns it back to a variable' (this is simplistic view which isn't really true, but will do for now). So you could, if you wanted (which you wouldn't) do this *&z which would be the same as doing z.

Basically think of & and * as a pair of complimentary operators like - and + or * and /. (They aren't operators, but to start with it might help).

The next problem is when you do this


All function parameters in C/C++ are passed by value by default. This means the function gets a copy of them not the actual value. So if you had:

void FunctionOne (int val)
    val = 99;
int val = 4;
printf("val = %d", val);   // You will see 4 printed.

If you pass by reference then you will see 99 printed, to pass by reference you can do it one of two ways.

void FunctionTwo (int& valByRef);
void FunctionThree (int* valPointer);

FunctionTwo only works with C++ (It will work with an Arduino).

FunctionThree might cause a segmentation error if NULL is passed in and the function writes to it.

If it helps in my opinion pointers was one of the hardest concepts to grasp in classic C/C++ so don't worry if you don't understand it just have a play with it and see what you can do.

Calling Methods

  • object.method() - Call the function method on the instance called object which is of type X.
  • object->method() - Call the function method on the instance called object which is a pointer of type X.
  • object::method() - call the function method which is a static member of class object (static means it doesn't change any data within an instance of a class)
  • *object->method() - Avoid this like the plague. It either means *(object->method()) or (*object)->method() to know you would need to look at the type of object and the return type of the function. Its easier just to bracket it to remove ambiguity.

As for *object.&method() I've never come across that one, but that doesn't mean it doesn't exist. :)

| improve this answer | |
  • Those who don't understand pointers are doomed to program in Javascript the rest of their lives. – user31481 Jan 29 '18 at 19:39
  • Thank you very much for this explanation. Good explanation! What confuses me is: Sometimes you can access a property by object.method or by object.object.method then by object::method and object->method or object.&method or *object.&object::method and object.@->/!%&wtf$method and so on. These things were not made for human brains to be understood. – Palmstroem Jan 29 '18 at 20:00
  • Are you saying I'm not human? :) (See edit) Its learning a new language, and you just have to remember certain rules, the more you use them the more they become ingrained, there is no short cut :( – Code Gorilla Jan 31 '18 at 1:07
  • Thank you very much. Superhuman! Any ideas for the original question? I'm inside a signal-handler-function and want to access an attribute of a parent or grand-parent class, but don't know how to find the right method. The documentation at felis.github.io/USB_Host_Shield_2.0/… seems to be quite comprehensive. But I don't get the clue. – Palmstroem Feb 1 '18 at 18:11
  • !Hid->GetAddress() always returns 0. Maybe I need to do something like *evt.HIDDevice.Hid->GetAddress() but all trials like that lead to compile errors. – Palmstroem Feb 1 '18 at 18:20

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