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Unit 6 IC INTERFACING
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. TTL driving CMOS When interfacing different types of ICs, we must check that the driving device can meet the current and voltage requirements of the load device. CMOS TTL Parameter 4000B 74HC 74HCT 74 74LS 74AS 74ALS VIH(min) VIL(max) VOH(min) VOL(max) 3.5V 1.5V 4.95V 0.05V 2.0V 0.8V 4.9V 0.1V 2.0V 0.8V 2.7V 0.5V 3.5V 1.0V 4.9V 0.1V 2.0V 0.8V 2.7V 0.4V 2.0V 0.8V 2.7V 0.5V 2.0V 0.8V 2.7V 0.5V 1uA 1uA 4mA 4mA IIH(max) IIL(max) IOH(max) IOL(max) 40uA 20uA 200uA 20uA 1.6mA 0.4mA 2mA 100uA 0.4mA 0.4mA 2mA 400uA 16mA 8mA 20mA 8mA 1uA 1uA 0.4mA 0.4mA 1uA 1uA 4mA 4mA *Supply voltage=5V +CMOS driving only CMOS inputs. Table 1: Worst-case values for CMOS/TTL Interfacing
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.
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.
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
Solution: The 74als series has 1lL(max) =100uA Thus, the 74HC can drive forty 74als inputs (4mA/100uA=40).
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.
3.3 High-voltage CMOS driving TTLCMOS 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.