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74F402
TABLE 2.
Applications
In addition to polynomial selection there are four other
capabilities provided for in the 74F402 ROM. The first is set
or clear selectability. The sixteen internal registers have the
capability to be either set or cleared when P
is brought
LOW. This set or clear capability is done in four groups of 4
(see Table 2, P
0
–P
3
). The second ROM capability (C
0
) is in
determining the polarity of the check word. As is the case
with the Ethernet polynomial the check word can be
inverted when it is appended to the data stream or as is the
case with the other polynomials, the residue is appended
with no inversion. Thirdly, the ROM contains a bit (C
1
)
which is used to select the RFB input instead of the SEI
input to be fed into the LSB. This is used when the polyno-
mial selected is actually a residue (least significant) stored
in the ROM which indicates whether the selected location
is a polynomial or a residue. If the latter, then it inhibits the
RFB input.
As mentioned previously, upon a successful data transmis-
sion, the CRC register has a zero residue. There is an
exception to this, however, with respect to the Ethernet
polynomial. This polynomial, upon a successful data trans-
mission, has a non-zero residue in the CRC register (C7 04
DD 7B)
16
. In order to provide a no-error indication, two
ROM locations have been preloaded with the residue so
that by selecting these locations and clocking the device
one additional time, after the last check bit has been
entered, will result in zeroing the CRC register. In this man-
ner a no-error indication is achieved.
With the present mix of polynomials, the largest is 56
th
order requiring four devices while the smallest is 16
th
order
requiring just one device. In order to accommodate multi-
plexing between high order polynomials (X 16
th
order) and
lower order polynomials, a location of all zeros is provided.
This allows the user to choose a lower order polynomial
even if the system is configured for a higher order one.
The 74F402 expandable CRC generator checker contains
6 popular CRC polynomials, 2-16
th
Order, 2-32
nd
Order, 1-
48
th
Order and 1-56
th
Order. The application diagram
shows the 74F402 connected for a 56
th
Order polynomial.
Also shown are the input patterns for other polynomials.
When the 74F402 is used with a gated clock, disabling the
clock in a HIGH state will ensure no erroneous clocking
occurs when the clock is re-enabled. Preset and Master
Reset are asynchronous inputs presetting the register to S
or clearing to 1s respectively (note Ethernet residue and
56
th
Order select code 8, LSB, are exceptions to this).
To generate a CRC, the pattern for the selected polynomial
is applied to the S inputs, the register is preset or cleared
as required, clock is enabled, CWG is set HIGH, data is
applied to D input, output data is on D/CW. When the last
data bit has been entered, CWG is set LOW and the regis-
ter is clocked for n bits (where n is the order of the polyno-
mial). The clock may now be stopped if desired (holding
CWG LOW and clocking the register will output zeros from
D/CW after the residue has been shifted out).
To check a CRC, the pattern for the selected polynomial is
applied to the S inputs, the register is preset or cleared as
required, clock is enabled, CWG is set HIGH, the data
stream including the CRC is applied to D input. When the
last bit of the CRC has been entered, the ER
output is
checked: HIGH = error free data, LOW = corrupt data. The
clock may now be stopped if desired.
To implement polynomials of lower order than 56
th
, select
the number of packages required for the order of polyno-
mial and apply the pattern for the selected polynomial to
the S inputs (0000 on S inputs disables the package from
the feedback chain).
Select Code
P
3
P
2
P
1
P
0
C
2
C
1
C
0
Polynomial
0 0000100S = 0
C 1111101Ethernet
D 1111101Polynomial
E 0000000Ethernet
F 0000010Residue
7 1111100CRC-16
B 1111100CRC-CCITT
3 1111100
2 111110056th
4 1111100Order
8 0011100
5 111110048th
9 1111100Order
1 1111100
6 111110032nd
A 1111100Order
