6.002 CIRCUITS ANDELECTRONICS Violating the Abstraction Barrier... dt Ldi Drop across inductor Inverter current v inductor solution 1.. avoid big current swings VS VS... Case 3: The Doub
Trang 16.002 CIRCUITS AND
ELECTRONICS
Violating the Abstraction Barrier
Trang 2Case 1: The Double Take
i
V
O
V
“0” Æ “1”
t
O
V
“0”
“1”
t
O
V
“0”
“1”
observed expected
in forbidden region!
huh?
Trang 3(a) DC case
R
i V
O
very high impedance, like open circuit
OK
DC V
5
Vi = VO = 5 V DC V1 = 5 V DC
Trang 4V 5 2
=
O
V
t
i
V
(b) Step
not ok!
t
1 V
looks ok!
b.1
b.3
b.2
V 0
V 5
0
=
t
0
=
t
=
T
2
V 5
V 5
R
i V
O
very high impedance, like open circuit
O
V
Trang 5i
V
instantaneous R divider finite propagation speed
of signals
characteristic
impedance
→
R
T
5
2.5
0
V 5 0
V
5
0 V
5
Trang 6Question: So why did our circuits work?
More in 6.014
t
V 5 V 5 2
3 Termination
Parallel termination
DEMO add R at th
e end
0
VO
2 Keep wires short
t
V 5
0
0
VO
DEMO use small w
ire
“Source Termination
”
1 Look only at V1
t
V 5
0
V1
DEMO
Trang 7Case 2: The Double Dip
Problem Æ strange spikes on supply
driving a 50 Ω
resistor!
V
0
1
OK
Why?
input
driving a 50 Ω
resistor!
V
0
Trang 8dt Ldi
Drop across inductor
Inverter current
v inductor
solution 1 short wires
2 low inductance wires
3 avoid big current swings
VS
VS
Trang 9Case 3: The Double Team, or,
Slower may be faster!
Problem
a given chip worked,
but was slow.
Let’s try speeding it up by using stronger drivers
actual ideal
ideal
C
Disaster!
L
ω
actual
Trang 10Consider
crosstalk!
1
R
0
R
DEMO
2
R
C
dt
dV
α
DEMO
ok
dt dV C
Trang 11How does this relate to chip?
Load output! — put cap on outputs of chip
— jitter edges
— slew edges
dt
dV
small DEMO
Solution
Trang 12Case 4: The Double Jump
Careful abstraction violation for the
better…
Recall
o
V
i
V
o
V
i
V
expect
but, observe
o
V
i
V
Trang 13Case 4: The Double Jump
Careful abstraction violation for the
better…
i
V
V 5
5V 0V
3V 5V + 3V So, pullup hasstronger drive
as output rises