Datasheet

Data Sheet ADAS1000-3/ADAS1000-4
Rev. A | Page 49 of 76
CRC Word
Framed data integrity is provided by CRCs. For the 128 kHz
frame rates, the 16-bit CRC-CCITT polynomial is used. For the
2 kHz and 16 kHz frame rates, the 24-bit CRC polynomial used.
In both cases, the CRC residue is preset to all 1s and inverted
before being transmitted. The CRC parameters are summarized
in Table 23. To verify that data was correctly received, software
should compute a CRC on both the data and the received
checksum. If data and checksum are received correctly, the
resulting CRC residue should equal the check constant shown
in Table 23. Note that data is shifted through the generator
polynomial MSB first, the same order that it is shifted out
serially. The bit and byte order of the CRC that is appended to
the frame is such that the MSB of the CRC is shifted through
the generator polynomial first in the same order as the data so
that the CRC residue XOR’d with the inverted CRC at the end
of the frame is all 1s (which is why the check constant is
identical for all messages). The CRC is based only on the
data that is sent out.
Figure 73. Input Clock
Clocks
The ADAS1000-3/ADAS1000-4 run from an external crystal or
clock input frequency of 8.192 MHz. The external clock input is
provided for use in gang mode so conversions between the two
devices are synchronized. In this mode, the CLK_IO pin is an
output from the master and an input from the slave. To reduce
power, the CLK_IO is disabled when not in gang mode.
All features within the ADAS1000-3/ADAS1000-4 are a
function of the frequency of the externally applied clock. Using
a frequency other than the 8.192 MHz previously noted causes
scaling of the data rates, filter corners, ac leads-off frequency,
respiration frequency, and pace algorithm corners accordingly.
Table 23. CRC Polynomials
Frame Rate CRC Size Polynomial
Polynomial
in Hex Check Constant
2 kHz, 16 kHz 24 bits x
24
+ x
22
+ x
20
+ x
19
+ x
18
+ x
16
+ x
14
+ x
13
+ x
11
+ x
10
+ x
8
+ x
7
+ x
6
+ x
3
+ x
1
+ x
0
0x15D6DCB 0x15A0BA
128 kHz 16 bits x
16
+ x
12
+ x
5
+ x
0
0x11021 0x1D0F
XTAL1
XTAL2
ADAS1000-3/
ADAS1000-4
CLK_IO
10997-034