See http://youtu.be/C7e9xtzVvSw

button_driver.h

#ifndef BUTTON_DRIVER_H_
#define BUTTON_DRIVER_H_

interface button_interface
{
    void pressed(void);
};
    
void button_driver(in port button_port, client interface button_interface button_interface);
    
#endif

button_driver.xc

#include <xs1.h>
#include "button_driver.h"

#define DEBOUNCE_DELAY 10 // in ms

void button_driver(in port button_port, client interface button_interface button_interface)
{
    int current_val = 0;
    int is_stable = 1;
    timer t;
    unsigned debounce_timeout;

    while(1)
    {
        select
        {
            case is_stable => button_port when pinsneq(current_val) :> int new_val:
                if(!(new_val & 0x01))
                    button_interface.pressed();
                current_val = new_val;
                is_stable = 0;
                int current_time;
                t :> current_time;
                debounce_timeout = current_time + (DEBOUNCE_DELAY * XS1_TIMER_MHZ * 1000);
                break;
            case !is_stable => t when timerafter(debounce_timeout) :> void:
                is_stable = 1;
                break;
        }
    }
}

larsson.h

#ifndef LARSSON_H_
#define LARSSON_H_

#include <stdint.h>
    
enum larsson_mode
{   
    KITT,
    KARR
};  

typedef struct larsson_s
{
    int8_t dir;
    uint16_t current_index;
} larsson_s;
        
void larsson_init(larsson_s &this, uint8_t buf[], uint16_t len);
void larsson_update(larsson_s &this, uint8_t buf[], uint16_t len, enum larsson_mode mode);
    
    
#endif

larsson.xc

#include "larsson.h"

void larsson_init(larsson_s &this, uint8_t buf[], uint16_t len)
{
    this.current_index = 0;
    this.dir = 1;
    for(uint16_t i=0; i < len; i++)
    {
        buf[i] = 0;
    }
}

void larsson_update(larsson_s &this, uint8_t buf[], uint16_t len, enum larsson_mode mode)
{
    for(uint16_t i=0; i < len; i++)
    {
        buf[i] >>= 1;
    }
    buf[this.current_index*3+1] = 0xff;
    if(mode == KARR) buf[this.current_index*3] = 0xff;
    if((this.dir == 1 && this.current_index*3 == len-3) || (this.dir == -1 && this.current_index*3 == 0))
    {
        this.dir *= -1;
    }
    this.current_index += this.dir;
}

led_strip_scanner.xc

#include <xs1.h>
#include <platform.h>

#include "ws2811_driver.h"
#include "button_driver.h"

out buffered port:4 led_strip_port = XS1_PORT_1A;
in port button_port = XS1_PORT_32A;


int main(void)
{
    interface button_interface i_button;
    par
    {
        on tile[0]: ws2811_driver(led_strip_port, i_button);
        on tile[0]: button_driver(button_port, i_button);
    }
    return 0;
}

ws2811_driver.h

#ifndef WS2811_DRIVER_H_
#define WS2811_DRIVER_H_

#include "button_driver.h"

void ws2811_driver(out buffered port:4 p, server interface button_interface button_interface);

#endif

ws2811_driver.xc

#include <xs1.h>
#include <platform.h>
#include <stdint.h>

#include "button_driver.h"
#include "larsson.h"

#define LED_COUNT 40*3      // 75 RGB LEDs (3 colours)
//#define UPDATE_PERIOD 3000  // Update every 3000 ms
#define UPDATE_PERIOD 50  // Update every 50 ms

timer t;

clock led_clock = XS1_CLKBLK_1;
void ws2811_driver(out buffered port:4 p, server interface button_interface button_interface)
{
    configure_clock_rate(led_clock, 100, 32);
    configure_out_port(p, led_clock, 0);
    start_clock(led_clock);

    uint8_t led_buffer[LED_COUNT];
    larsson_s s;
    larsson_init(s, led_buffer, LED_COUNT);
    enum larsson_mode mode = KITT;

    uint32_t time;
    t :> time;
    for(uint8_t i=0; i<200; i++) p<:0b0001;
    while(1)
    {
        [[ordered]]
        select
        {
          case t when timerafter(time) :> void:
              for(uint16_t i=0; i<LED_COUNT; i++)
              {
                  for(uint8_t j=0; j<8; j++)
                  {
                      if((led_buffer[i] >> (7-j)) & 0x01)
                      {
                          p <: 0b0011;
                      }
                      else
                      {
                          p <: 0b0001;

                      }
                  }
              }
              p <: 0b0000; // reset
              larsson_update(s, led_buffer, LED_COUNT, mode);
              time += UPDATE_PERIOD * XS1_TIMER_MHZ * 1000;
              break;
          case button_interface.pressed(void):
              mode = (mode == KITT) ? KARR : KITT;
              break;
        }
    }
}