Output current

1
Output current
IO is the current that the device can source at
its output. In this case, 15 mA is the
guaranteed minimum.
Some devices have two numbers, IOH for a logic
1 andIOL for a logic 0
The logic convention for all currents is into the
device
2
Input Current
IIH is the current that the device sinks with a
logic 1 at the input (20 mA). IIL is the
current that the device can sink with a logic 0
at the input. (-0.1 mA)
The logic convention for all currents is into the
device
3
Fan-Out
Fan-out is the minimum of two ratios IOH / IIH
the number of devices the gate can drive with a
1 out and IOL / IIL the number of devices
the gate can drive with a 0 out Here
15mA/20mA750
4
Numbers in fixed-length registers

• Numbers in a fixed-length register can be
  interpreted as
• positive integers only, or as
• signed integers
• Regardless of the system used, the internal
  representation and mathematical operations are
  unchanged. The interpretation is entirely in the
  mind of the user
• The number of bits in a register determines the
  range of numbers that can be represented

5
Binary Number System

• In a binary number system, the binary digits
  (bits) represent powers of 2x 11012 is read
  as 123 122 021 120 1310
• When a signed integer interpretation is used,
  twos complement is used to represent negative
  numbers.Here the most significant bit (MSB) is
  a sign bit and represents whether the number is
  negative (MSB 1), or whether the number is
  non-negative (MSB 0)

.
6
Twos complement

• The twos complement obeys the rules of the minus
  sign
• 2s2sxx -(-x)x
• A-BA 2sB A - B A (-B)

7
Range of a register

• For an n-bit register, the range of positive
  integers is 0 ? 2n - 1
• For an n-bit register, the most positive integer
  in a signed number interpretation is2n-1 - 1

8
A special case

• The number 100000 is a special case
• 2s1000000 1000000and it may be
  interpreted to mean -x x ? x0and 1000 can
  be called - 0
• BUT, x 1000000 ? x

9
A special case, part 2

• If 100000 is interpreted as - 2 n-1
• it obeys x 1000000 x - 2 n-1
• BUT, - (1000000) ? 2 n-1
• Despite the 2d condition, 100000 is usually
  interpreted as - 2 n-1

10
Overflow

• Overflow can only occur when
• two positive numbers are added and the result is
  a negative number, or
• two negative numbers are added and the result is
  a positive number(Here, subtraction is treated
  as the addition of the negative).

1xxxxx 0xxxxxx 1xxxxx 0xxxxx
0xxxxxx
1xxxxx ?xxxxx
1xxxxxx 0xxxxx Cannot
overflow overflow overflow
11
Homework for Tuesday, 9/22
1) Given a 6-bit register, what is the range of
the register a) When numbers are interpreted as
positive integers? b) When numbers are
interpreted as signed integers? 2) Given an
8-bit register and the number 1111 0000 a) What
is the decimal equivalent when interpreted as a
positive integer? b) What is the decimal
equivalent when interpreted as a signed
integer? 3) Given an 8-bit register a) What is
the representation of the decimal number 127? b)
What is the representation of the decimal number
-100? 4) Show the decimal equivalents of the
following arithmetic operations and indicate
whether or not overflow has occurred. Interpret
numbers as signed integers.
001111 010000 101111 101111
010101 - 011000
12
5) Show the connections to convert the binary
counter shown below into a decade counter. That
is, 0000?0001 ?... ?1001 ?0000 ?. Use
additional gates as needed
13
6) Show the connections to make an 8-bit binary
counter. Be sure to label the outputs as Q0, Q1,
, Q7
14
7) Show the connections to make an 8K x 16 EPROM
15
8) What is the fan-out of the device shown below
when driving identical gates?