Hardware Reference
In-Depth Information
In the
group-copy-mode
,anN -bit slice is divided into M
D
N=K groups, and
each group is K-bits wide with the possible exception for the last group. If a group
contains more than two target-symbols, the group-copy-mode is used and the en-
tire group is copied to a data-code. Two data-codes are needed to encode a group.
The first data-code specifies the index of the first bit of the group, and the second
data-code contains the actual data. In the group-copy-mode, don't-care bits can be
randomly filled instead of being mapped to 0 or 1 by the compression scheme. For
example, let N
D
8 and K
D
4, i:e: each slice is 8-bits wide and consists of two
4-bit groups. To encode the slice “X1110000”, the three 1s in group 0 are encoded.
The resulting data-codes are “0000” and “X111”, which refer to bit 0 (first bit of
group 0) and the content of the group, respectively.
Since data-codes are used in both modes, control-codes are needed to avoid ambi-
guity. Control-codes “00”, “01” and “10” are used in the single-bit-mode and “11”
is used in the group copy-mode. Control-codes “00” and “01” are referred to as
initial control-codes
and they indicate the start of a new slice. Table
7.1
shows a
complete example to illustrate the encoding procedure. The first column shows the
scan slices. The second and third ones show the resulting slice-codes (control- and
data-codes) and the last column describes the compression procedure.
A property of this compression method is that consecutive c-bit compressed
slices fed by the ATE are often identical or compatible. Therefore, ATE pattern-
repeat can be used to further reduce test data volume after selective encoding of
scan slices. More details about ATE pattern-repeat can be found in
Wa n g a n d
Chakrabarty (
2005
).
7.6.2.2
Test Power Considerations
The above technique drastically reduces test data volume (up to 28x for a set of
experimented industrial circuits) and test time (up to 20x). However, power con-
sumption is not carefully considered, especially during the filling of don't-care bits
in the scan slices. To illustrate this problem, let us consider the 4 slice-code example
given in Table
7.2
with N
D
8 and K
D
2.
Tabl e 7. 1
A slice encoding - example 1
Slice Codes:
Slice Codes:
Slices
Descriptions
Data Code
Control Code
XX00 010X
00
0101
Start a new slice, map Xs to 0, set
bit 5 to 1
1110 0001
00
0111
Start a new slice, map Xs to 0, set
bit 7 to 1
11
0000
Enter group-copy-mode starting
from bit 0
11
1110
The data is 1110
XXXX
XX11
01
1000
Start a new slice, map Xs to 1, no
bits are set to 0
