Hardware Reference
In-Depth Information
D
1=2:S
i
:F
i
:C
0
:V
DD
E
i
(7.3)
According to this formulation, the energy consumed after application of a pair of
successive input vectors (V
k
-
1
, V
k
) can be expressed by:
E
Vk
D
1=2:C
0
:V
DD
:
X
i
S
i
.k/:F
i
(7.4)
Where i ranges across all the nodes of the circuit and S
i
.k/ is the number of transi-
tions provoked by V
k
at node i .Now,the
total energy
consumed in the circuit after
application of the complete test sequence of length L is given below, where k ranges
across all the vectors of the test sequence.
D
1=2:C
0
:V
DD
:
X
k
X
E
total
S
i
.k/:F
i
(7.5)
i
By definition, power is given by the ratio between energy and time. The
instanta-
neous power
is generally calculated as the amount of power required during a small
instant of time t
small
such as the portion of a clock cycle immediately following the
system clock rising or falling edge. Consequently, the instantaneous power dissi-
pated in the circuit after the application of a test vector V
k
can be expressed by:
P
inst
.V
k
/
D
E
Vk
=t
small
(7.6)
The
peak power
corresponds to the highest value of instantaneous power measured
during test. It can be expressed in terms of the highest energy consumed during a
small instant of time during the test session:
P
peak
D
Max
k
P
inst
.V
k
/
D
Max
k
.E
Vk
=t
small
/
(7.7)
Finally, the
average power
consumed during the test session can be calculated from
the total energy and the test time. Considering that the test time is given by the
product L:T ,whereT corresponds to the nominal clock period of the circuit, the
average power can be expressed as follows:
P
average
D
E
total
=. L : T /
(7.8)
The above expressions of power and energy, although based on a simplified model,
are accurate enough for the intended purpose of power analysis during test. Accord-
ing to these expressions, and assuming a given technology and a supply voltage for
the considered circuit, it appears that the switching activity factor S
i
is the only pa-
rameter that has impact on the energy, peak power, and average power. This explains
why most of the methods proposed so far for reducing power and/or energy during
test are based on a reduction of the switching activity factor.
