Violating The Abstraction Barrier

6.002

CIRCUITS AND ELECTRONICS

http://electrical.globalautomation.info

Violating the Abstraction Barrier

Case 1: The Double Take

Problem

R

VO

“0” Æ “1” Vi

expected

observed

VO “1”

VO “1” huh?

“0”

t

t “0” in forbidden region!

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(a) DC case R

VO

V1

Vi

Vi = 5V DC

VO = 5V DC V1 = 5V DC

very high impedance, like open circuit

OK

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(b)

Step R

VO

V1

Vi

very high impedance, like open circuit

5V Vi b.1

0V b.3

5V

t

t=0 VO

not ok!

VO = 2.5V t =0 b.2

2T

t

5V V1 looks ok!

t=0 T

t

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2.5

....

R 5

R→

Vi

characteristic impedance

instantaneous R divider finite propagation speed of signals

5V

5V

5V 0

0

2T

0 T

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Question: So why did our circuits work?

5V V1 rce u o S “ on” i t a n i T e rm

1. Look only at V1

0

O M E D

2. Keep wires short O e DEM mall wir s us e

3. Termination O DEM at the R add e nd

0

t

T

5V VO

0

0

le l l a r a P on i t a n i term

5 V VO 2.5V

t

0

t

More in 6.014

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Case 2: The Double Dip

Problem Æ strange spikes on supply

0 V

1

1

0

OK driving a 50 Ω resistor!

0 V

driving a 50 Ω resistor!

input

Why? http://electrical.globalautomation.info

Drop across inductor Ldi dt

VS V

Inverter current

v inductor VS solution

1. short wires 2. low inductance wires 3. avoid big current swings

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Case 3: The Double Team, or, Slower may be faster! Problem

a given chip worked, but was slow. ideal

C

actual

Let’s try speeding it up by using stronger drivers ideal

ω L

actual

Disaster! http://electrical.globalautomation.info

Why?

DEMO

Consider

ok

C

DEMO

R1

R0

R2

dV α dt dV C dt

crosstalk! http://electrical.globalautomation.info

How does this relate to chip?

Solution

DEMO

small

Load output!

dV dt

— put cap on outputs of chip — jitter edges — slew edges http://electrical.globalautomation.info

Case 4: The Double Jump

Careful abstraction violation for the better… Recall

Vo expect

Vi

Vo Vi but, observe

Vo Vi

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Case 4: The Double Jump

Careful abstraction violation for the better…

5V

Vi

5V + 3V

5V 0V

3V

So, pullup has stronger drive as output rises

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