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Objectives
Unit 6
1. Introduction
• Interfacing means connecting the output (s) of
one circuit or system to the input (s) of another
circuit or systems that has different electrical
characteristics.
• A direct connection is often not possible of the
difference in electrical characteristic of the
driver circuit that is providing the output signal
and the load circuit that is receiving the signal.
1. Introduction
• An interface circuit is a circuit connected between
the driver and the load. Its function is to take the
driver output signal and condition it so that it is
compatible with the requirements of the load.
• ICs from the same logic series are designed to be
connected together without any special
considerations, provided the fan-out limitation of
each output is not exceeded.
1. Introduction
• When the output of an IC is connected to the input
of an IC of the different logic family or a different
series within the same logic family, the voltage and
current parameters of the two devices will have to be
taken care of.
2.When
TTL
driving CMOS
interfacing different types of ICs, we must check that the driving device can
meet the current and voltage requirements of the load device.
CMOS
Parameter
4000B
TTL
74HC
74HCT
74
74LS
74AS
74ALS
2.0V
0.8V
2.7V
0.5V
2.0V
0.8V
2.7V
0.5V
2.0V
0.8V
2.7V
0.4V
VIH(min)
VIL(max)
VOH(min)
VOL(max)
3.5V
1.5V
4.95V
0.05V
3.5V
1.0V
4.9V
0.1V
2.0V
0.8V
4.9V
0.1V
2.0V
0.8V
2.7V
0.5V
IIH(max)
IIL(max)
IOH(max)
IOL(max)
1uA
1uA
0.4mA
0.4mA
1uA
1uA
4mA
4mA
1uA
1uA
4mA
4mA
40uA
20uA 200uA
1.6mA 0.4mA 2mA
0.4mA 0.4mA 2mA
16mA 8mA
20mA
*Supply voltage=5V
+CMOS driving only CMOS inputs.
Table 1: Worst-case values for CMOS/TTL Interfacing
20uA
100uA
400uA
8mA
2. TTL driving CMOS
Table 1 shows that the input current values for
CMOS are extremely low compared to the output
current capabilities of any TTL series.
Thus, TTL has no problem meeting the CMOS input
current requirements.
Comparing the TTL output voltages with the CMOS
input Voh (min) of every TTL series is too low when
compared to the Vih (min) requirements of the 400B
and 74HC series.
2. TTL driving CMOS
For these situations, something must be done to raise the
Fig.6.1 shows the solution for interfacing TTL and
CMOS ICs, where the TTL output is connected to +5 V
with a pull-up resistor.
The pull-up resistor will cause the TTL output to raise to
approximately +5V in the high state, thereby providing
an adequate CMOS input voltage.
A value of 1k (ohms) to 10k (ohms) for the resistor is
normally used.
2.1 TTL driving 74HCT
Since the 74HCT series is designed so that it can be
driven directly by TTL outputs, its input voltage
requirements are the same as standard TTL devices.
There is no need for external pull-up resistor.
+5v
Rp
TTL
CMOS
Fig 6.1 External pull-up resist or used when TTL drives CMOS
2.2 TTL driving high- voltage CMOS
Assuming the CMOS is operating at Vdd= +10 V, the
CMOS input high-state voltage required will be Vih
(min) = +7V, ( 70% of Vdd).
The outputs of many TTL devices cannot operate at
more than +5 v, so a pull-up resistor connected to
+10V is prohibited.
In this case, a 7407 open-collector buffer is used as
there interface between a TTL output and the CMOS
operating at Vdd greater than +5 v.
.2 TTL driving high-voltage CMOS
+10V
+5V
1K
74LS112
---
J
---
>CLK
---
K
CMOS
Q
7407 ---
-Q
Fig 6 . 2 A 7407 open – collector buffer used to interface TTL to high - voltage CMOS
3. CMOS driving TTL
Before considering the interfacing problems of CMOS
output to TTL inputs, let’s review the CMOS output
characteristic for the two logic states.
3 .1
CMOS driving TTL in the HIGH state.
Table 1 shows that CMOS output can easily supply
enough voltage (Voh) to satisfy the TTL input
requirement in the HOIGH state (Vih).
CMOS output can supply more than enough current
(Iih) to meet the TTL input current requirements(Iih).
Thus, no special consideration is neede in the HIGH
state.
3. 2 CMOS driving TTL in the low state
Table 1 shows that TTL inputs have a relatively high
input current in the LOW state, ranging from 100A to 2
m.
The 74HC and 74HCT families can sink up to 4mA, no
problem driving a single TTL load of any series.
The 4000B series is, however, more limited.
Its low Iol capability is not sufficient top drive even one
input of the 74 or 74AS series.
Example 1
How many 74LS inputs can be driven by a 734HC
output? Repeat for a 4000B otput.
Given: Iil ( max )+ 0.4mA for 74LS
Iol( max) = 4mA for 74HC
Iil ( max )= 0.4mA for 400B
Solution:
The 74LS series has Iil (max )=0.4mA
The 74HC can sink up to Iol (max) =4mA
Thus, the 74HC can drive ten 74LS
loads.(4mA/0.4mA=10)
The400B can sink only 0.4mA, so it can drive only one 74LS
input.
Example 2:
How many 74ALS inputs can be driven by a 74HC output?
Repeat for a 400B output.
Given: Iil (max) = 100uA for 74ALS
The 74als series has 1lL(max) =100uA
Thus, the 74HC can drive forty 74als inputs
(4mA/100uA=40).
74HC00
74AS00
A
B
74LS04
---
---
C
4001and 6.4(b)?
What is wrong with the circuit in fig 6.4(a)
A
74AS00
B
---
D
fIG 6.4(a)
---
---
C
Fig 6.4(b)
74AS04
74LS00
74LS04
---
In Fig 6.4(a), the 74HC00 can sink 4mA, but the three 74AS inputs require
3x2mA=6mA
In Fig 6.4(b), the 4001B can sink 0.4mA, but the three 74LS inputs require
3x0.4mA=1.2mA
For situations like these, some type of interface circuits which has a low input current
requirement and a sufficiently high output current rating is required to drive the loads.
4001B
A
4050B
-----
B
---
4001B
74LS125
A
---
B
---
74LS inputs
(a)
---
74LS inputs
(b)
Fig 6.5 Buffers used to interface low-curre nt CMOS to 74LS inputs
In fig 6.5(a), a CMOS 4050B non-inverting b uffer that has an output current rating
of 1oL(max)=3mAis used to drive the three 7 4LS loads directly.
In fig 6.5(b), the 74LS125 is a permanently enabled non-inverting tri-state
buffer,driven directly by the 4001B,to driv e the 74LS loads.
In both cases, the interface buffer simply passes the 4001B output signal to the
74LS loads.
3.3 High-voltage CMOS driving TTL
CMOS Ics operating at high-voltage (e.g. Vdd=+15V)
will have a high state output voltage close to Vdd.
Most TTL inputs cannot handle more than 7V and so an interface is necessary if
they are to be driven from high-voltage CMOS.
The interface functions as a voltage-level translator that converts the high-voltage
input to a 5V output that can be connected to TTL.
Fig 6.6 shows how the 4050B performs this level translation between 15V and 5V.
Vdd=+15V
15V --0
A
B
4001B
+5V
5V --0
4050B C
74LS00
----
Fig 6.6 A 4050B buffer serving as a level translator
between high-voltage CMOS and TTL.