I’m working on the SPI communication between a microcontroller and the L9963E. The datasheet of the L9963E shows little information about the CRC calculation, but mentions:
- a CRC of 6 bits,
- a polynomial of X6 X4 X3 1 =
0b1011001 - a seed of
0b111000
The documentation also mentions in the SPI protocol details that the CRC is a value between 0x00 and 0x3F, and is "calculated on the [39-7] field of the frame", see Table 22.
I'm wondering: What is meant by "field [39-7]"? The total frame length is 40bits, out of this the CRC makes up 6 bits. I would expect a CRC calculation over the remaining 34 bits, but field 39-7 would mean either 33 bits (field 7 inclusive) or 32 bits (excluding field 7).
Since I have access to a L9963E evaluation board, which includes pre-loaded firmware, I have hooked up a spectrum analyzer. I have found the following example frames to be sent to the L9963E from the eval-board, I am assuming that these are valid, error-free frames.
0xC21C3ECEFD
0xC270080001
0xE330081064
0xC0F08C1047
0x827880800E
0xC270BFFFF9
0xC2641954BE
Could someone clear up the datasheet for me, and maybe advise me on how to implement this CRC calculation?
CodePudding user response：
All of the special frames have CRC == 0:
(0000000016 xor E000000000) mod 59 = 00
(C1FCFFFC6C xor E000000000) mod 59 = 00
(C1FCFFFC87 xor E000000000) mod 59 = 00
(C1FCFFFCDE xor E000000000) mod 59 = 00
(C1FCFFFD08 xor E000000000) mod 59 = 00
and as noted by Mark Adler only one of the messages in the question works:
(C270BFFFF9 xor E000000000) mod 59 = 00
CodePudding user response：
Bit ranges in datasheets like this are always inclusive.
I suspect that this is just a typo, or the person who wrote it temporarily forgot that the bits are numbered from zero.
Looking at the other bit-field boundaries in Table 19 of the document you linked it wouldn't make sense to exclude the bottom bit of the data field from the CRC, so I suspect the datasheet should say bits 39 to 6 inclusive.
There is a tool called pycrc that can generate C code to calculate a CRC with an arbitrary polynomial.
CodePudding user response：
If a 40-bit message is in the low bits of uint64_t x;, then this:
x ^= (uint64_t)0x38 << 34;
for (int j = 0; j < 40; j )
x = x & 0x8000000000 ? (x << 1) ^ ((uint64_t)0x19 << 34) : x << 1;
x &= 0xffffffffff;
gives x == 0 for all of the example messages in the document. But only for one of your examples in the question. You may not be extracting the data correctly.