George Zhu’s BLOG

Here's some code to initialize the accelerometer to run at full speed. Pass the desired sample rate (in Hz) to the initialize function. Go wild!

#include <IOKit/IOKitLib.h>
#include <CoreFoundation/CoreFoundation.h>

typedef struct {} *IOHIDEventSystemRef;
typedef struct {} *IOHIDEventRef;
float IOHIDEventGetFloatValue(IOHIDEventRef ref, int param);

void handleHIDEvent(int a, int b, int c, IOHIDEventRef ptr) {
    int type = IOHIDEventGetType(ptr);
    if (type == 12) {
        float x,y,z;
        x = IOHIDEventGetFloatValue(ptr, 0xc0000);
        y = IOHIDEventGetFloatValue(ptr, 0xc0001);
        z = IOHIDEventGetFloatValue(ptr, 0xc0002);
        // do whatever you need to do with the gravity
        ballSetAccel(x, y);
    }
}

#define expect(x) if(!x) { printf("failed: %s\n", #x); return; }

void initialize(int hz) {
    mach_port_t master;
    expect(0 == IOMasterPort(MACH_PORT_NULL, &master));

    int page = 0xff00, usage = 3;

    CFNumberRef nums[2];
    CFStringRef keys[2];
    keys[0] = CFStringCreateWithCString(0, "PrimaryUsagePage", 0);
    keys[1] = CFStringCreateWithCString(0, "PrimaryUsage", 0);
    nums[0] = CFNumberCreate(0, kCFNumberSInt32Type, &page);
    nums[1] = CFNumberCreate(0, kCFNumberSInt32Type, &usage);
    CFDictionaryRef dict = CFDictionaryCreate(0, (const void**)keys, (const void**)nums, 2, &kCFTypeDictionaryKeyCallBacks, &kCFTypeDictionaryValueCallBacks);
    expect(dict);

    IOHIDEventSystemRef sys = (IOHIDEventSystemRef) IOHIDEventSystemCreate(0);
    expect(sys);

    CFArrayRef srvs = (CFArrayRef)IOHIDEventSystemCopyMatchingServices(sys, dict, 0, 0, 0);
    expect(CFArrayGetCount(srvs)==1);

    io_registry_entry_t serv = (io_registry_entry_t)CFArrayGetValueAtIndex(srvs, 0);
    expect(serv);

    CFStringRef cs = CFStringCreateWithCString(0, "ReportInterval", 0);
    int rv = 1000000/hz;
    CFNumberRef cn = CFNumberCreate(0, kCFNumberSInt32Type, &rv);

    int res = IOHIDServiceSetProperty(serv, cs, cn);
    expect(res == 1);

    res = IOHIDEventSystemOpen(sys, handleHIDEvent, 0, 0);
    expect(res != 0);
}

Original URL: Medallia Blog