Department of Mechanical Engineering

MEMS1082 Chapter 6 Digital Circuit 6-7

Department of Mechanical Engineering

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Special Purpose Digital ICs Decade Counter

Can be constructed by using a 7490IC. This is a negative edge-triggered counter, and the output is in binary coded decimal (BCD) consisting of 4 bits As input pulses arrive, the units BCD counter advances one count per pulse. On the tenth input pulse the units counter recycles to zero, causing its flip-flop D output to go from 1 to 0. This 1-to-0 transition acts as the clock input for the tens counter and causes it to advance one count

Department of Mechanical Engineering

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Special Purpose Digital ICs Decade Counter

As additional pulses occurs, unit counter advances one counter per pulse, and tens counter advances when units counter recycles to zero; and so on to 0099. One the hundreds input pulse, the units and tens counter recycles to 0, thus the flip-flop D output of the tens counters making a 1-to-0 transition, which acts as the clock input for the hundreds counter to advance one count………

Department of Mechanical Engineering

Special Purpose Digital ICs

Department of Mechanical Engineering

Special Purpose Digital ICs Decade Counter

Department of Mechanical Engineering

Special Purpose Digital ICs 7447 decoder driver to convert the 4BCD bits at its

inputs into 7-bit code to drive the LED seven segments

Open-collector output is used to drive the LED. Thus when the 7447 outputs are LOW, the LED is ON

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Special Purpose Digital ICs Schmitt Trigger: is not a flip-flop, but it does exhibit a type of

memory characteristic that makes it useful in certain special situations

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Special Purpose Digital ICs Schmitt Trigger SPST debounce circuit

When the switch is closed, and the 7414's output goes high. Depending on the capacitor value, the capacitor voltage might discharge below the low threshold of the 7414 on first contact. If not, the capacitor will eventually discharge as any bouncing occurs and continual contact is established. When the switch is opened, the capacitor again begins to charge. Any switch bounce occurring when the switch is opened just momentarily interrupts the charging. When the charge on the capacitor exceeds the high threshold of the 7414, the output again goes low. The result is a clean, bounce-free, output pulse.

a Schmitt trigger RC circuit can be used to debounce an SPST switch.

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Special Purpose Digital ICs 555 Timer: 555IC is known as “time machine”. It can be used

in bounce-free switch, cascaded timers, frequency dividers, voltage-controlled oscillators, pulse generators………

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Special Purpose Digital ICs For monostable multivibrator

External resistor and capacitor

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Special Purpose Digital ICs 555 Timer for monostable multivibrator

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Special Purpose Digital ICs When the circuit is powered up or after the reset is activated, the output

will be low (Q = 0), transistor Ql is saturated (ON) shorting capacitor C, and the outputs of both comparators are low.

When the trigger pulse goes below (1/3)Vcc, the trigger comparator goes high, setting the flip-flop. Now the output is high (Q = 1), transistor Ql cuts off, and the capacitor begins to charge with time constant τ = RaC. The time constant is the time required for the capacitor voltage to reach 63.2% of its full charge value Vcc.

When the capacitor voltage reaches (2/3)Vcc. the threshold comparator resets the flip-flop, which resets the output low (Q = 0) and discharges the capacitor again. If the trigger is pulsed while the output is high, it has no effect. The length of the pulse is given approximately by

∆T~ 1.1 RaC