The 74LS90 has one independent toggle JK flip-flop driven by the CLK A input and three toggle JK flip-flops that form an asynchronous counter driven by the CLK B input as shown. The 74LS90 consists of four master-slave JK flip-flops internally connected to provide a MOD-2 (count-to-2) counter and a MOD-5 (count-to-5) counter.
The 74LS90 integrated circuit is basically a MOD-10 decade counter that produces a BCD output code. The main advantage of BCD code is that it allows for the easy conversion between decimal and binary forms of numbers. The 8421 designation refers to the binary weight of the four digits or bits used. Note also that a digital counter may count up or count down or count up and down (bidirectional) depending on an input control signal.īinary-coded-decimal code is an 8421 code consisting of four binary digits. Then a single stage BCD counter such as the 74LS90 counts from decimal 0 to decimal 9 and is therefore capable of counting up to a maximum of nine pulses. It is called a BCD counter because its ten state sequence is that of a BCD code and does not have a regular pattern, unlike a straight binary counter. Once the count value is reached, resetting them returns the counter back to zero to start again.īut as we saw in the Asynchronous Counters tutorial, that a counter which resets after ten counts with a divide-by-10 count sequence from binary 0000 (decimal “0”) through to 1001 (decimal “9”) is called a “binary-coded-decimal counter” or BCD Counter for short and a MOD-10 counter can be constructed using a minimum of four toggle flip-flops. That is from 0 to 9 in decimal and fortunately for us, counting circuits are readily available as integrated circuits with one such circuit being the Asynchronous 74LS90 Decade Counter.ĭigital counters count upwards from zero to some pre-determined count value on the application of a clock signal. To make a digital counter which counts from 1 to 10, we need to have the counter count only the binary numbers 0000 to 1001. But the problem with 4-bit binary counters is that they count from 0000 to 1111. Then a flip-flop counter has two possible output states and by adding more flip-flop stages, we can make a divide-by-2 N counter. If we connect together several toggle flip-flops in a series chain we can produce a digital BCD counter which stores or display the number of times a particular count sequence has occurred.Ĭlocked T-type flip-flops act as a binary divide-by-two counter and in asynchronous counters, the output of one counting stage provides the clock pulse for the next stage. We saw previously that toggle T-type flip flops can be used as individual divide-by-two counters.
0 kommentar(er)
