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The 74LS90 Decade Counter

الكلية كلية الهندسة     القسم  الهندسة الكهربائية     المرحلة 4
أستاذ المادة ايهاب عبد الرزاق حسين محمد       16/12/2016 19:09:18
The 74LS90 Decade Counter

The 74LS90 is a 4-bit asynchronous, negative edge-triggered decade counter with asynchronous clear and present inputs for programmable counter applications.The 74LS90 counts only in an ascending sequence. The IC actually consists of two separate counters that can be configured for three different modes of operation. The two internal counters are a MOD-2 counter and a MOD-5 counter, providing a total of four output stages.

Mode 1:
The MOD-2 and MOD-5 counters operate separately with individual clock inputs. The MOD-2 counter toggles from a logic HIGH to a logic LOW state on each clock pulse. The MOD-5 counter produces a 3-bit count sequence from 0 to 4. Figure (25) shows the 74LS90 in this configuration, along with the resulting waveforms and state transition diagrams.

Mode 2:
The MOD-2 counter operates as the LSB of a 4-bit decade counter producing a BCD count sequence. The control clock is input into the clock A input, and the QA output is input to clock B to form the cascaded connection. This MOD-2 to MOD-5 cascaded counter configuration produces a 4-bit BCD count sequence of 0 to 9. Figure (26) shows the 74LS90 connected in this configuration and the output waveforms.

Mode 3:
The MOD-2 counter operates as the MSB of a 4-bit decade counter. The control clock is input into clock B and the QD output is input into clock A. This is referred to as the biquinary counter since it produces two five-state count sequences: In the first five sequences of 0 to 4 the most significant output, QA, is a logic LOW; in the last five sequences of 0 to 4 the most significant output, QA, is logic HIGH. The resulting set of output waveforms are with the QA output having a 50% duty cycle, while the three lower order outputs, QD, Qc, and QB, cycle through the 3-bit sequence of 0 to 4. Figure (27) shows the 74LS90 in the biquinary configuration with the output wave-forms.

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