Information Technology Reference
In-Depth Information
Dynamic behavior of the original inverter versus an inverter with reduced cI:operator binding
P
Z
orig
P
Z
modified
0.02
0.01
0
0
10
20
30
40
50
60
70
6
x 10
3
P
Z
A
2
orig
P
Z
A
2
modified
4
2
0
0
10
20
30
40
50
60
70
0.02
P
Z
A
4
orig
P
Z
A
4
modified
0.01
0
0
10
20
30
40
50
60
70
0.01
mRNA
Z
orig
mRNA
Z
modified
0.005
0
0
10
20
30
40
50
60
70
time (x100 sec)
Figure 4.11
Improving gate delay: the effect of reducing the repressor binding effi-
ciency by a factor of 100. For the promoter and output mRNA, the graphs compare
the molecular concentrations of the original gate and of the modified gate. Overall,
the modification reduces the gate delay in switching from low to high output.
discussed above. The simulations provide a first indication of the feasibility of
using the proposed chemical reactions to implement logic circuits. Furthermore,
the analysis and simulations of modifications to the kinetic coefficients provide
valuable insights toward reliable system design.
Storage: Analysis of an RS Latch
The RS latch is a good initial test circuit for the biochemical inversion model and
should operate correctly even if its constituent parts are not perfectly matched.
It persistently maintains a data bit that can be toggled o
n
and
off
. The RS latch
consists of two cross-coupled NAND gates, with inputs
S
and
R
for setting and
resetting the
c
omplementary output values
A
and
B
(Figure 4.12). The inverters
with inputs
R
and
B
and comm
on
output
A
constitute one of the NAND gates,
while the inverters with input
s
S
an
d
A
and common output
B
constitute the
other NAND gate. The inputs
S
and
R
are normally high, and are set to low to
toggle the latch.
Figure 4.13 shows the correct simulated dynamic behavior of this RS
l
atch
in
response to relatively short and long input pulses. Initially, both inputs
S
and
R
are high, and the outputs
A
and
B
compete for dominance. In this simulation,
