Matrix (i,j) entry shows USB usage code
------
Row is J1 connector; column is j2
------

    1  2  3  4  5  6  7  8  9 10 11
    -----------------------------------
 1  68
 2  69
 3    6A 6B
 4    6C 6D
 5          6E 6F
 6          70 71
 7                72 73
 8                87 88 
 9                89 8A
10                      8B 8C 8D 8E
11                      8F 90 91 92
12                      93 94 95 96
13                      97 98 80 81
14
15
16

Keybinding Usage code --> foot key virtual keycode
----------

static struct {
  unsigned char  usage_code; 
  unsigned short foot_key;} foot_key_table[32] = {
    // independent group number one: shift keys
    0x68, FOOT_VK_SHIFT_L,
    0x69, FOOT_VK_SHIFT_R,
    // independent group number two: control keys
    // assumption: FOOT_VK_CTRL_L and FOOT_VK_ALPHA_CTRL_L are not depressed
    // simultaneously, the same thing holds for the right keys
    0x6a, FOOT_VK_CTRL_L,
    0x6b, FOOT_VK_ALPHA_CTRL_L,
    0x6c, FOOT_VK_CTRL_R,
    0x6d, FOOT_VK_ALPHA_CTRL_R,
    // independent group number three: alt keys
    // assumption: FOOT_VK_ALT_L and FOOT_VK_ALPHA_ALT_L are not operated
    // simultaneously; the same thing holds for the right keys
    0x6e, FOOT_VK_ALT_L,
    0x6f, FOOT_VK_ALPHA_ALT_L,
    0x70, FOOT_VK_ALT_R,
    0x71, FOOT_VK_ALPHA_CTRL_R,
    // independent group number four: function keys
    // assumption: at most one of FOOT_VK_F_CTRL_L, FOOT_VK_F_L, and
    // FOOT_VK_F_ALT_L is activated at the same time; similar
    // assumption for the right keys
    0x72, FOOT_VK_F_CTRL_L,
    0x73, FOOT_VK_F_CTRL_R,
    0x87, FOOT_VK_F_L, 
    0x88, FOOT_VK_F_R,
    0x89, FOOT_VK_F_ALT_L,
    0x8a, FOOT_VK_F_ALT_R,
    // independent group number 5: normal keys
    0x8b, VK_ESCAPE,
    0x8c, VK_TAB,
    0x8d, VK_UP,
    0x8e, VK_PRIOR,
    0x8f, VK_BACK,
    0x90, VK_DOWN,
    0x91, FOOT_VK_MOUSE_LEFT,
    // 0x92 can be generated, but is not used
    0x93, VK_SPACE,
    0x94, VK_DELETE,
    0x95, VK_RETURN,
    0x96, VK_NEXT,
    0x97, VK_LEFT,
    0x98, FOOT_VK_MOUSE_RIGHT,
    0x80, VK_RIGHT,
    // 0x81 may be generated, but is not used
  };