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Old Nov 01, 2012, 01:38 PM
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Quote:
Originally Posted by PhracturedBlue View Post
To automate it, I'd need to hookup a digital power-gate to the power supply, and to drive the transmission in a more automated fashion. I should be able to rig something up pretty easily, but it'll take some time. I'll see what I can do.
Funny I was thinking about the same thing. I was thinking using some ESC
like xp-3a to flash it provide on and off as needed. Actually, all you need is a
NFET I think. If you can find pin on the Devo8 to drive that NFET, then the
receiver's ground is hook up to the NFET's sink. It should provide power gate
as you said.
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Old Nov 01, 2012, 01:45 PM
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I take a stab at the problem. Let's first try to solve how the channel are selected.
It will be a sub problem of the mapping.

I run some program to analyse the first 0x63 = 97 sequence. (I mistake the 0x63
as dec numbers).

Here is what I get if I sort the sequence of the channels:
You can already see, the distance between each channel is has only 4 values:
02, 04, 06, 08. (Edit, I found 10 and 12 in the next table as well, might be wrap around).


Code:
 01:  04  06  12  14  16  18  22  24  30  34  38  40  42  44  46  54  56  62  64  66  68  70  72
 02:  03  05  07  11  15  17  25  27  29  35  39  41  43  49  51  57  61  63  65  69  71  73  75
 03:  06  08  10  12  14  22  24  26  28  34  36  38  42  44  46  54  56  58  60  68  70  72  74
 04:  03  05  07  09  11  13  25  27  29  31  35  41  43  47  51  53  57  63  65  67  71  73  75
 05:  04  10  12  14  16  18  22  24  28  30  32  34  36  40  44  54  56  58  60  64  66  68  74
 06:  05  07  09  11  15  19  23  25  31  33  39  43  45  49  51  55  59  61  63  65  69  71  73
 07:  04  12  14  18  20  22  24  26  28  32  38  44  46  48  50  52  56  58  60  62  64  68  70
 08:  03  05  07  09  15  19  21  25  29  33  37  41  45  47  49  53  59  61  63  65  67  71  75
 09:  06  10  12  14  16  20  22  24  28  32  40  42  44  46  48  52  54  58  60  64  66  70  72
 10:  05  09  11  13  15  17  21  25  29  31  33  39  41  45  49  53  55  57  61  63  65  69  73
 11:  06  08  10  14  16  18  22  26  28  32  34  36  40  48  50  54  56  60  64  66  68  70  72
 12:  03  07  09  11  13  17  23  27  29  35  39  41  45  49  51  53  57  59  61  63  69  71  75
 13:  04  08  12  14  18  20  24  26  28  30  34  36  38  42  48  52  54  58  62  64  66  68  74
 14:  03  07  09  11  19  21  23  25  35  37  39  41  43  45  49  53  55  57  59  67  71  73  75
 15:  04  06  08  10  12  16  18  20  28  30  36  38  40  44  50  52  54  56  58  62  70  72  74
 16:  05  07  09  13  17  21  23  25  29  31  33  35  41  47  51  55  59  63  65  67  69  71  73
 17:  04  06  08  12  16  20  24  26  36  38  42  44  46  48  50  52  54  56  58  62  64  68  74
 18:  05  07  09  11  15  19  21  23  29  37  39  41  43  45  47  55  57  61  63  65  67  69  75
 19:  04  10  12  14  16  20  22  26  28  30  32  34  38  42  48  54  56  58  60  62  68  72  74
 20:  09  11  15  17  19  21  23  25  29  31  33  35  41  43  45  53  55  59  63  65  69  73  75
 21:  04  06  10  16  18  20  24  26  38  40  42  44  46  48  50  52  56  60  64  66  70  72  74
 22:  03  05  07  09  13  15  17  23  29  35  37  39  41  47  49  55  57  59  61  65  67  69  73
 23:  04  12  14  16  18  22  24  26  28  34  36  38  42  44  50  52  54  58  60  62  64  72  74
 24:  07  09  11  17  19  23  25  27  31  33  35  39  41  47  51  53  55  57  59  63  65  67  71
 25:  04  06  10  12  18  20  24  26  36  38  42  44  46  48  50  52  56  58  62  64  66  68  72
 26:  03  05  07  09  11  13  15  23  29  35  37  39  41  43  49  55  57  61  63  65  67  73  75
 27:  06  08  12  14  16  18  20  26  30  34  38  40  44  46  48  52  56  58  60  62  66  68  72
 28:  05  07  11  15  19  21  25  27  33  35  37  41  43  45  51  53  57  59  61  63  65  69  71
 29:  04  10  12  14  18  22  24  26  32  34  38  42  46  48  50  56  58  60  64  68  70  72  74
 30:  03  05  09  11  13  17  25  27  29  33  35  45  47  49  51  53  55  61  65  69  71  73  75
 31:  08  10  12  14  16  22  24  26  28  32  34  36  42  46  48  52  56  58  60  68  70  72  74
 32:  03  07  09  11  13  21  23  25  31  33  39  41  43  45  49  53  55  57  59  63  69  71  73
 33:  06  10  12  14  16  20  22  26  28  30  32  40  42  46  48  52  54  58  60  64  66  68  72
 34:  03  05  07  13  15  21  23  25  31  37  43  45  47  49  51  53  55  59  61  67  69  71  73
 35:  04  06  10  16  18  20  22  24  28  30  32  34  36  38  48  52  54  58  62  66  68  70  74
 36:  03  09  11  13  17  23  25  27  31  33  37  39  43  47  51  53  57  59  61  63  65  71  73
 37:  04  10  14  18  20  22  24  26  30  32  34  38  42  44  48  56  58  60  64  66  68  70  72
 38:  05  07  09  17  19  23  25  27  31  35  39  41  43  45  49  53  57  61  63  65  67  73  75
 39:  04  06  08  14  16  20  24  26  28  30  32  34  36  44  46  54  56  58  62  64  66  72  74
 40:  05  07  09  15  17  21  23  27  29  31  37  39  41  43  51  55  57  59  61  67  69  71  73
 41:  04  06  08  12  14  16  20  22  30  32  36  38  40  44  50  52  56  58  62  66  68  70  72
 42:  03  05  07  09  13  17  19  25  29  31  33  39  43  45  49  53  55  57  59  63  69  71  73
 43:  06  08  12  16  18  22  24  26  28  32  38  42  46  48  50  54  56  58  60  66  70  72  74
 44:  03  05  07  15  17  19  21  27  29  35  39  43  47  49  51  53  57  59  63  67  69  73  75
 45:  08  10  12  14  16  20  22  26  28  32  34  38  40  42  46  52  54  60  62  64  66  68  74
 46:  03  11  17  19  21  23  25  27  31  35  37  41  47  49  51  55  57  61  63  65  67  69  71
 47:  04  06  08  14  18  20  24  26  34  36  38  40  42  46  48  52  54  56  60  62  64  66  70
 48:  03  05  07  09  11  17  19  25  33  35  39  43  47  49  51  53  57  61  63  65  67  69  71
 49:  06  10  14  16  18  22  24  26  28  30  32  36  38  42  50  52  54  56  60  66  68  72  74
 50:  03  05  09  11  15  19  21  23  29  31  37  41  43  45  49  53  57  59  65  67  71  73  75
 51:  04  06  08  10  14  20  22  24  30  32  36  38  40  46  50  52  56  60  62  64  70  72  74
 52:  03  09  11  13  15  19  25  27  29  31  39  41  45  49  51  55  57  59  61  65  67  69  71
 53:  06  10  14  16  18  20  24  26  28  36  40  44  46  48  50  52  54  56  60  64  66  70  72
 54:  05  07  13  17  19  21  23  25  33  35  37  39  45  49  51  55  57  59  61  67  69  71  73
 55:  04  06  08  10  12  16  18  26  30  38  40  44  46  48  50  52  58  60  64  66  68  70  74
 56:  05  09  11  15  17  21  23  25  35  37  39  41  45  47  49  55  59  63  65  67  69  71  75
 57:  08  10  12  14  16  18  20  24  28  34  36  40  42  46  50  54  56  60  62  64  66  68  70
 58:  03  09  11  13  21  23  25  27  31  33  37  39  41  43  45  55  57  61  63  65  67  71  73
 59:  06  08  14  16  18  20  24  26  28  30  32  34  36  40  50  52  54  56  60  62  66  72  74
 60:  05  07  11  17  19  21  25  27  29  33  35  37  39  45  47  53  55  59  61  63  65  71  75
 61:  06  08  10  14  20  22  24  26  28  30  32  40  44  46  50  54  58  64  66  68  70  72  74
 62:  03  05  09  13  15  19  25  27  33  37  39  41  45  47  51  57  59  61  65  69  71  73  75
 63:  06  12  14  16  18  22  24  26  28  30  34  36  38  48  50  52  54  56  60  62  64  68  74
 64:  07  11  13  15  17  21  25  27  29  31  35  37  41  47  49  55  57  59  61  63  69  71  75
 65:  04  06  08  12  14  16  20  26  28  32  34  38  40  42  46  52  54  58  60  62  66  70  74
 66:  03  09  13  15  17  19  23  27  29  31  33  35  47  49  51  53  55  57  61  65  67  69  75
 67:  06  08  10  14  16  18  22  24  28  34  36  40  42  44  48  52  60  62  64  66  68  70  72
 68:  05  07  15  19  21  23  25  27  29  37  41  43  45  49  51  53  55  61  63  65  69  73  75
 69:  04  08  10  14  18  20  22  24  28  30  32  36  38  42  50  52  56  58  60  64  68  70  74
 70:  03  07  09  11  15  19  23  27  31  39  41  43  47  49  51  53  57  59  61  65  67  69  73
 71:  04  06  08  14  16  22  24  26  28  30  32  34  38  44  48  56  58  60  62  68  70  72  74
 72:  03  05  09  11  15  19  23  27  33  35  37  39  45  49  51  53  55  57  59  61  67  69  75
 73:  06  08  10  12  14  16  18  20  28  32  34  36  40  46  48  52  56  60  64  66  68  72  74
 74:  03  09  11  13  17  19  23  25  29  33  39  41  45  49  51  53  55  59  61  63  65  69  75
 75:  04  08  10  12  16  18  22  26  28  30  36  38  44  48  50  52  54  56  58  62  66  68  70
 76:  04  06  10  12  18  20  22  26  32  34  36  38  40  42  48  56  58  62  64  66  70  72  74
 77:  03  05  09  11  13  17  25  27  29  31  33  35  41  47  49  53  59  61  63  69  71  73  75
 78:  04  06  10  12  14  16  18  24  32  36  40  42  44  46  48  52  56  58  62  64  68  70  74
 79:  03  05  07  09  11  17  23  27  29  33  35  39  41  45  47  53  55  61  63  65  69  71  75
 80:  04  06  08  10  16  18  20  24  30  34  38  42  44  46  48  52  54  56  58  66  68  72  74
 81:  04  08  14  16  18  22  24  26  30  32  38  40  42  44  48  56  58  60  62  64  68  70  72
 82:  05  09  15  17  19  21  25  27  29  31  33  35  41  45  49  57  61  65  67  69  71  73  75
 83:  06  08  10  12  16  18  20  26  30  32  34  36  44  46  50  54  56  58  62  66  68  70  74
 84:  03  05  09  13  17  19  21  27  31  33  35  37  41  45  47  53  55  59  63  65  67  69  75
 85:  04  10  12  14  16  20  24  26  30  32  38  44  46  48  50  52  54  56  60  62  64  68  74
 86:  07  09  11  13  15  19  21  27  29  31  33  39  41  47  49  55  57  61  65  67  69  73  75
 87:  04  06  10  12  18  20  22  26  28  30  34  36  46  48  50  56  58  62  64  66  68  72  74
 88:  07  09  11  15  17  19  25  27  29  31  33  37  41  45  51  53  55  61  63  65  69  71  75
 89:  06  14  16  18  20  22  24  26  28  36  38  40  42  44  48  52  54  60  62  64  70  72  74
 90:  03  07  09  15  19  21  25  27  29  31  37  39  41  49  51  53  55  57  59  61  67  69  73
 91:  04  06  08  16  18  20  22  26  30  36  38  42  44  46  48  52  56  62  66  68  70  72  74
 92:  03  05  09  11  13  19  21  25  35  39  41  43  45  47  49  55  59  61  63  67  71  73  75
 93:  06  08  12  14  16  18  22  26  30  34  36  38  40  42  48  52  54  58  60  62  64  66  74
 94:  03  09  11  13  15  19  21  27  29  37  39  43  47  49  51  55  59  61  63  67  71  73  75
 95:  04  08  10  12  14  16  22  26  28  30  32  34  36  42  44  54  56  58  62  64  68  72  74
 96:  03  05  07  09  11  15  17  25  31  33  35  39  41  43  49  53  61  63  67  69  71  73  75
 97:  04  10  14  16  20  22  24  26  28  32  36  38  42  44  48  52  54  56  60  62  66  68  74
printk is offline Find More Posts by printk
Last edited by printk; Nov 01, 2012 at 01:52 PM.
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Old Nov 01, 2012, 01:49 PM
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Joined Jul 2009
63 Posts
And Here is the gap between the closest channel of each sequence:
Code:
 01:  02  06  02  02  02  04  02  06  04  04  02  02  02  02  08  02  06  02  02  02  02  02
 02:  02  02  04  04  02  08  02  02  06  04  02  02  06  02  06  04  02  02  04  02  02  02
 03:  02  02  02  02  08  02  02  02  06  02  02  04  02  02  08  02  02  02  08  02  02  02
 04:  02  02  02  02  02  12  02  02  02  04  06  02  04  04  02  04  06  02  02  04  02  02
 05:  06  02  02  02  02  04  02  04  02  02  02  02  04  04  10  02  02  02  04  02  02  06
 06:  02  02  02  04  04  04  02  06  02  06  04  02  04  02  04  04  02  02  02  04  02  02
 07:  08  02  04  02  02  02  02  02  04  06  06  02  02  02  02  04  02  02  02  02  04  02
 08:  02  02  02  06  04  02  04  04  04  04  04  04  02  02  04  06  02  02  02  02  04  04
 09:  04  02  02  02  04  02  02  04  04  08  02  02  02  02  04  02  04  02  04  02  04  02
 10:  04  02  02  02  02  04  04  04  02  02  06  02  04  04  04  02  02  04  02  02  04  04
 11:  02  02  04  02  02  04  04  02  04  02  02  04  08  02  04  02  04  04  02  02  02  02
 12:  04  02  02  02  04  06  04  02  06  04  02  04  04  02  02  04  02  02  02  06  02  04
 13:  04  04  02  04  02  04  02  02  02  04  02  02  04  06  04  02  04  04  02  02  02  06
 14:  04  02  02  08  02  02  02  10  02  02  02  02  02  04  04  02  02  02  08  04  02  02
 15:  02  02  02  02  04  02  02  08  02  06  02  02  04  06  02  02  02  02  04  08  02  02
 16:  02  02  04  04  04  02  02  04  02  02  02  06  06  04  04  04  04  02  02  02  02  02
 17:  02  02  04  04  04  04  02  10  02  04  02  02  02  02  02  02  02  02  04  02  04  06
 18:  02  02  02  04  04  02  02  06  08  02  02  02  02  02  08  02  04  02  02  02  02  06
 19:  06  02  02  02  04  02  04  02  02  02  02  04  04  06  06  02  02  02  02  06  04  02
 20:  02  04  02  02  02  02  02  04  02  02  02  06  02  02  08  02  04  04  02  04  04  02
 21:  02  04  06  02  02  04  02  12  02  02  02  02  02  02  02  04  04  04  02  04  02  02
 22:  02  02  02  04  02  02  06  06  06  02  02  02  06  02  06  02  02  02  04  02  02  04
 23:  08  02  02  02  04  02  02  02  06  02  02  04  02  06  02  02  04  02  02  02  08  02
 24:  02  02  06  02  04  02  02  04  02  02  04  02  06  04  02  02  02  02  04  02  02  04
 25:  02  04  02  06  02  04  02  10  02  04  02  02  02  02  02  04  02  04  02  02  02  04
 26:  02  02  02  02  02  02  08  06  06  02  02  02  02  06  06  02  04  02  02  02  06  02
 27:  02  04  02  02  02  02  06  04  04  04  02  04  02  02  04  04  02  02  02  04  02  04
 28:  02  04  04  04  02  04  02  06  02  02  04  02  02  06  02  04  02  02  02  02  04  02
 29:  06  02  02  04  04  02  02  06  02  04  04  04  02  02  06  02  02  04  04  02  02  02
 30:  02  04  02  02  04  08  02  02  04  02  10  02  02  02  02  02  06  04  04  02  02  02
 31:  02  02  02  02  06  02  02  02  04  02  02  06  04  02  04  04  02  02  08  02  02  02
 32:  04  02  02  02  08  02  02  06  02  06  02  02  02  04  04  02  02  02  04  06  02  02
 33:  04  02  02  02  04  02  04  02  02  02  08  02  04  02  04  02  04  02  04  02  02  04
 34:  02  02  06  02  06  02  02  06  06  06  02  02  02  02  02  02  04  02  06  02  02  02
 35:  02  04  06  02  02  02  02  04  02  02  02  02  02  10  04  02  04  04  04  02  02  04
 36:  06  02  02  04  06  02  02  04  02  04  02  04  04  04  02  04  02  02  02  02  06  02
 37:  06  04  04  02  02  02  02  04  02  02  04  04  02  04  08  02  02  04  02  02  02  02
 38:  02  02  08  02  04  02  02  04  04  04  02  02  02  04  04  04  04  02  02  02  06  02
 39:  02  02  06  02  04  04  02  02  02  02  02  02  08  02  08  02  02  04  02  02  06  02
 40:  02  02  06  02  04  02  04  02  02  06  02  02  02  08  04  02  02  02  06  02  02  02
 41:  02  02  04  02  02  04  02  08  02  04  02  02  04  06  02  04  02  04  04  02  02  02
 42:  02  02  02  04  04  02  06  04  02  02  06  04  02  04  04  02  02  02  04  06  02  02
 43:  02  04  04  02  04  02  02  02  04  06  04  04  02  02  04  02  02  02  06  04  02  02
 44:  02  02  08  02  02  02  06  02  06  04  04  04  02  02  02  04  02  04  04  02  04  02
 45:  02  02  02  02  04  02  04  02  04  02  04  02  02  04  06  02  06  02  02  02  02  06
 46:  08  06  02  02  02  02  02  04  04  02  04  06  02  02  04  02  04  02  02  02  02  02
 47:  02  02  06  04  02  04  02  08  02  02  02  02  04  02  04  02  02  04  02  02  02  04
 48:  02  02  02  02  06  02  06  08  02  04  04  04  02  02  02  04  04  02  02  02  02  02
 49:  04  04  02  02  04  02  02  02  02  02  04  02  04  08  02  02  02  04  06  02  04  02
 50:  02  04  02  04  04  02  02  06  02  06  04  02  02  04  04  04  02  06  02  04  02  02
 51:  02  02  02  04  06  02  02  06  02  04  02  02  06  04  02  04  04  02  02  06  02  02
 52:  06  02  02  02  04  06  02  02  02  08  02  04  04  02  04  02  02  02  04  02  02  02
 53:  04  04  02  02  02  04  02  02  08  04  04  02  02  02  02  02  02  04  04  02  04  02
 54:  02  06  04  02  02  02  02  08  02  02  02  06  04  02  04  02  02  02  06  02  02  02
 55:  02  02  02  02  04  02  08  04  08  02  04  02  02  02  02  06  02  04  02  02  02  04
 56:  04  02  04  02  04  02  02  10  02  02  02  04  02  02  06  04  04  02  02  02  02  04
 57:  02  02  02  02  02  02  04  04  06  02  04  02  04  04  04  02  04  02  02  02  02  02
 58:  06  02  02  08  02  02  02  04  02  04  02  02  02  02  10  02  04  02  02  02  04  02
 59:  02  06  02  02  02  04  02  02  02  02  02  02  04  10  02  02  02  04  02  04  06  02
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Old Nov 01, 2012, 03:04 PM
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Quote:
Originally Posted by printk View Post
Funny I was thinking about the same thing. I was thinking using some ESC
like xp-3a to flash it provide on and off as needed. Actually, all you need is a
NFET I think. If you can find pin on the Devo8 to drive that NFET, then the
receiver's ground is hook up to the NFET's sink. It should provide power gate
as you said.
Well, it is easier to do with a PFet, but yes that is my thought. however, I think I'm going to try to implement it on my Raspberry Pi. it is easier to program for, and since it is a Tx-only setup, there are minimal frequency/latency requirements on the SPI. We'll see which way is easier (there are some GPIO pins pinned out on the Devo Tx, so I could go that way)
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Old Nov 01, 2012, 10:56 PM
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I run a statistic of the gap between channel. There is the distribution:
2: 1290
4: 564
6: 202
8: 62
10: 13
12: 3

Then I run a simulation random select 23 channel from the 39 channel.
Here is the distribution of the gap between channels:
2: 1244
4: 541
6: 211
8: 91
10: 32
12: 7
14: 5
16: 3

So the channel selections are pretty close to random. With slightly bias towards
spreading the channel out and avoiding the gap too big.

I think it is very likely DSMX using the guid as seed to select channels from a
pseudo random generator. Because each telemetric receiver is also a TX.
It need a big sequence space to avoid collision. That makes it very hard
to reverse engineer. For small sample data, they pretty much as same
as random noise. It will be too expensive to enumerate the whole space.
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Old Nov 02, 2012, 02:00 AM
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I also run a distribution of the channel jump number.
The channel jump define as the distance between the previous channel.
The X is the channel jump distance. Y is the total number of this channel
jump:


Again, it is almost a perfect triangle except 0. The channel can not
stay in the same place without jmping. That match pretty close to
the random sequence.

So it is pretty obvious DSMX is using some kind of pseudo random
sequence to perform the channel jumping. Since it need to cover the
P(39, 23) without start to repeat itself. This pseudo random generator
need to have a lot of internal state bits. The more internal state, the longer
the cycle of the pseudo random generator start to repeat itself. Which
means it is harder to guess what is internal state of the pseudo random
generator. That assume you already know what is the formula of
the pseudo number generator, just guess the "seed" of the generator
is hard enough. This internal state obviously need to has more bits than
32 bit.

Assume HH do the right things to pick the good random number generator.
It will be impossible to break by looking at the sequence it produce.
You can not do better than randomly guess a sequence and find out
if it match or not.

I have a feeling that this is going to be extremely hard if possible at all.
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Old Nov 02, 2012, 11:24 AM
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Thanks for the analysis. So, unless we can extract the firmware from a DSMX Rx/Tx, we should probably not pursue this further.
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Old Nov 02, 2012, 01:00 PM
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I thinks its pretty obvious that the GUID is a seed to a random number generator in both the TX and RX firmware. I only collected about 32 sequences so far but I noticed that the channel range seems to be limited between 3 and 75 for a total of 72 channels used. PhracturedBlue, your list also seems to have the same limits.

So, a random number generator that takes a 32 bit seed that can be limited to all odd or all even results between 3 and 75? That's sounds like more than just a random number generator at work. I think it would be safe to say that there are probably some rules or checks applied to each result from the generator to ensure they meet the given range. Is the fact that even GUIDs produce odd channels and vice-versa intentional or is it a side effect? I think you also need to be mindful of the fact that the random number routine is running on an 8 bit microprocessor with limited RAM (CY8C21434). And this chip does have a seedable psuedo random sequence generator module in it. It uses a linear feedback shift register architecture. I think I might begin playing around with LFSRs.

PhracturedBlue,
Did you ever try using a GUID value of 0000h? What were the results? I think I'll try that when I get home tonight.
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Old Nov 02, 2012, 04:06 PM
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Thanks for your reply, Hammer. I was going to ask your opinion since
you make some TX modules.

First of all, if you already have the random number generator provide
long enough sequence without repeating, design such a channel skipping
is not hard. Check and make sure it meet the range means some of the
random bits are going to be wasted. In general is not very good.
I can easily design some thing eliminate that with very high probability .

Secondly, the 23 byte channel sequence only need to calculate once
per binding sequence. Usually new binding will take up to 30s to complete
any way. So that 8 bit micro controller can have enough resource to do
reasonable long cycle random number calculation. Do you know how
many bit in the build in psuedo random sequence generator module ?
Once the sequence is calculated. The MCU can store it in the EEPROM
with the rest of the binding information like guid etc.

The even and odd is obvious by design and not a side effect of the
random number generator. You can build a function that generate random
skip sequence of 23 channel out from 36. Then just assign the 36 odd channel
number to even guild, even channel to odd guid. That part is easy.
There is no rules to check and retry here.

Select 23 channel sequence from 36 can be construct with very few
wasted random bits (and retry) as well. I notice the DSMX channel
selection does not produce gaps more than 6 channel. So it is likely
they use some segmented way to select channels. More to that
later.



Quote:
Originally Posted by hammer22 View Post
So, a random number generator that takes a 32 bit seed that can be limited to all odd or all even results between 3 and 75? That's sounds like more than just a random number generator at work. I think it would be safe to say that there are probably some rules or checks applied to each result from the generator to ensure they meet the given range. Is the fact that even GUIDs produce odd channels and vice-versa intentional or is it a side effect? I think you also need to be mindful of the fact that the random number routine is running on an 8 bit microprocessor with limited RAM (CY8C21434). And this chip does have a seedable psuedo random sequence generator module in it. It uses a linear feedback shift register architecture. I think I might begin playing around with LFSRs.

PhracturedBlue,
Did you ever try using a GUID value of 0000h? What were the results? I think I'll try that when I get home tonight.
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Last edited by printk; Nov 02, 2012 at 04:20 PM.
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Old Nov 02, 2012, 04:30 PM
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I think it is still interesting to dig more information about the random
number generator to get insign for how DSMX work. I am still interested
to build a automatic way to get the sequence out from the DSMX receiver.
Do you know the MCPX receiver speaks the DSMX with DX4e or not?
If it is, I should have enough parts to do the experiment.

Just like break encryption. Usually the way to break it is not brute force.
It is finding the weakness on it. We haven't know for sure DSMX
sequence has no weakness yet. e.g. The channel selection is not true
pseudo random. It is likely to hint what kind of the method was used
to produce that kind of the bias.

Quote:
Originally Posted by PhracturedBlue View Post
Thanks for the analysis. So, unless we can extract the firmware from a DSMX Rx/Tx, we should probably not pursue this further.
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Old Nov 02, 2012, 09:51 PM
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Hi printk,
According to the data sheet, the CY8C21434 chip can be configured for an 8, 16, 24, or 32 bit sequence generator. I'm no expert on the Cypress PSoC chips but it looks like they are similar to PAL chips in that they have blocks of digital and analog modules that get configured for a specific purpose (SPI, UART, timer, A to D, etc...) at programming time. I have to assume that some of these blocks are needed for interfacing with the CYRF6936 chip (SPI), the main TX CPU (serial) and timer functions. My guess, and probably not an accurate one, is that the LFSR is only 8 bits wide.
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Old Nov 03, 2012, 05:29 AM
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8 bit LFSR can only generate 256 bytes before it start to repeat itself.
It does not make sense to use only 8 bit LFSR.

The goal of the GUID to channel sequence mapping should satisfy the
property that, no two GUID should map to the same sequence.
Also, the minimal distance between the result sequence is bigger
the better. The sequence distance is define as how many non
common sub sequence.

Using only 8 bit LFSR will resulting random number cycle is too
small for this purpose.

Of course, if you keep adding feed backs of the LFSR bits from the
GUID, that effectually increase the internal state of the bits.

32 bit LFSR should be the minimal of this mapping.
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Old Nov 03, 2012, 06:12 PM
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I solder the MOSI, CLK, SS pin on the CYRF69103 chip
that is part of a totalled MCPX 3 in 1 board. The receiver
part of the board is fine and the MCU part is fine. Perfect
for this experiment.

PhracturedBlue and Hammer, is that all I need for snooping
the SPI bus? I already see signal using oscilloscope.
I will fire up the logic analyser soon.

I solder this without using the microscope. Got to be one of my
finest solder job recently.
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Old Nov 03, 2012, 06:22 PM
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Quote:
Originally Posted by printk View Post
I solder the MOSI, CLK, SS pin on the CYRF69103 chip
that is part of a totalled MCPX 3 in 1 board. The receiver
part of the board is fine and the MCU part is fine. Perfect
for this experiment.

PhracturedBlue and Hammer, is that all I need for snooping
the SPI bus? I already see signal using oscilloscope.
I will fire up the logic analyser soon.

I solder this without using the microscope. Got to be one of my
finest solder job recently.
You may also need MISO. You need to be able to see both the 0x21 command, and the response. On my 6210 Rx, it is in 3-wire mode, so I don't need it, but on the Transmitter board from my Walkera, I need both.
Good job on the soldering. I've done it a couple times now, and keep breaking the wires off over time (and last time, I managed to from my 6210Rx). I'm currently using wire probes held in place with a jury-rigged micro-manipulator which works well, but requires delicate alignment.

I finally got my RPi up and running with SPI. I'm just starting to experiment to see if I can get accurate enough timing to communicate with a DSM Rx.
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Old Nov 03, 2012, 10:57 PM
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The CYRF69103 internally only use 3 wire SPI. I load up the
logic analyser and it capture the SPI data fine. Now I need to
read the datasheet to understand what it is dowing.
I think the trick for avoiding the wire break off the pad is that,
it need to use very thin wire. I am using 33 gauge wire. I don't
have smaller one. Also make sure the pin has good wetting
before the soldering.
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