Interfacing between CMOS and TTL

Mumbai University > ELECTRO > Sem 3 > Digital Circuits and Designs

Marks: 5M

Year: Dec 2014

1 Answer
  • When we interface different types of ICs, it is necessary to check whether the driving IC is capable of meeting the current and voltage requirements of the load IC or not.

  • For example the table below reveals that the output current capability of TTL Ics is much higher than the input current values of CMOS ICs.

- 4000B 74HC/HCT 74 74LS 74AS
$I_{IH(max)}$ 1μA 1μA 40μA 20μA 20μA
$I_{IL(max)}$ 1μA 1μA 1.6mA 0.6mA 0.5mA
$I_{OH(max)}$ 0.4mA 4mA 0.4mA 0.4mA 2mA
$I_{OL(max)}$ 0.4mA 4mA 16mA 8mA 20mA
  • Therefore there is no problem for TTL to drive CMOS as far as current is concerned.

  • But there is a problem when we compare the voltage levels of TTL and CMOS. Because VOH of a TTL series is very low as compared with $V_{IH(min)}$ required for CMOS like 4000B, 74HC or 74HCT.

  • In such situations, something has to be done to raise the output voltage to an acceptable voltage level for CMOS.

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  • Due to the presence of pull up resistor, there will be rise in the TTL output to approximately +5V in the HIGH state.

  • This will provide sufficient voltage level at the input of CMOS gate. Such a pull up resistor is not needed if the CMOS gate belongs to 74 HCT or 74 ACT family since these families can accept the TTL outputs directly.

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