GCSE Electronics Component 2: Sequential Systems PDF

GCSE Electronics: Component 2 Unit 3: Sequential systems (Page 3) Combining counters and logic circuits BCD counters and seven-segment displays Decoder/driver ICs One of the most common uses for counters is to automate...

GCSE Electronics: Component 2 Unit 3: Sequential systems (Page 3) Combining counters and logic circuits BCD counters and seven-segment displays Decoder/driver ICs One of the most common uses for counters is to automate Whilst computers count in binary, for humans it is difficult An interface to link the BCD counter to a seven-segment display certain electronic systems that have a continuously repeating to translate a binary number such as 1010110102 into the is needed to convert the BCD output from the counter into pattern, e.g. traffic lights. A combinational logic circuit is equivalent decimal number (316) at a glance. To improve the appropriate logic signals to light the correct segment of the linked to a clock and counter to produce these fully automatic readability of a counter output for humans, we need: display. systems. i. a counter that follows a decimal count sequence, i.e. 0-9 The device performs two functions: For example, design an automatic traffic light display to produce the following sequence. ii. a decimal display device that is easy to read. It decodes the BCD counter outputs A, B, C and D into the seven input signals a, b, c, d, e, f, g required for the i. A counter that follows a decimal count system seven-segment display (this is the decoder part). Binary coded decimal It provides a current boost to the output signals to provide enough current to light the display segments directly (this is the Look at the following table. driver part). Single digit counting system with the BCD counter made from a binary counter The truth table for the system is shown below. Decoder / Driver D C B A The logic circuits are as follows The third column in the table is a modified version of binary, R called binary coded decimal (or BCD for short). In this case, the Red =BNOT B Red maximum count is restricted to 1001, i.e. 9 in decimal. After that, resetting occurs when the count reaches 1010. Yellow A =A Yellow A binary counter IC can be made into a BCD counter as shown Single digit counting system with a dedicated BCD counter below. IC manufacturers have produced a special version of the binary A Green = NOT A AND B Green counter which is internally wired to reset after a count of nine B D C B A D C B A has been reached. Using this device removes the need for the external AND gate to reset the counter. Using a BCD counter, R R By adding a counter and clock to the logic system, the the system will look like this: sequence will then automatically repeat itself. Note how NAND and AND gates are used depending on The full circuit diagram is shown below. whether the reset is active low (Logic 0) or active high (Logic 1) Red respectively. Decoder / Driver Yellow Green ii. Seven-segment displays D C B A The most common display for counting systems is BCD Counter D C B A the seven-segment display, a single package that R Clock R has seven LED’s arranged as shown opposite.  GCSE Electronics: Component 2 Unit 1: Sequential systems (Page 4) Creating other characters on a seven-segment display Segment b and c: These segments follow the output from the Decade counters game; therefore, segments ‘b’ & ‘c’ can be connected directly to The seven-segment display can also be used to display other the output from the game. A decade counter, as its name suggests, counts in decades or characters as shown below. tens; however, it is not a binary counter. It has one clock input Segment d: This segment is the opposite to the output from and ten outputs. Each output is activated in turn when the clock the game and therefore needs to be inverted. This can be pulses arrive. The diagram below shows a 4017 decade counter achieved by using an inverter between the output from the connected to a logic analyser. game and segment ‘d’. Segment e, f and g: These segments are always on whatever letter is being displayed; therefore, the ‘e’, ‘f’, & ‘g’ input can be connected permanently to the supply voltage line. The full solution is as follows. When we want to create these different characters, we cannot use a decoder/driver IC as this is pre-programmed to convert the output of a BCD counter into the numbers 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. Output 0 will go high when the circuit is switched on. On the rising edge of the first clock pulse, output 0 goes low and To create special characters, we would have to design the logic output 1 goes high. On the rising edge of the second clock decoder required ourselves. pulse, output 1 goes low and output 2 goes high and so on. For example, a seven-segment display is used to show the The output of the logic analyser shows what happens at the letter ‘T’ and ‘H’ in a ‘heads’ or ‘tails’ game. A logic 0 should output pins of the counter over the first two seconds after the display the letter ‘T’, and a logic 1 should display the letter ‘H’. circuit is switched on or the reset switch SW1 is momentarily Step 1: Complete a truth table for each condition. closed. Output a b c d e f g Display from game 0 0 0 0 1 1 1 1 Tt 1 0 1 1 0 1 1 1 H Step 2: Look at each segment in turn and compare it to the Ch. 0 shows the clock pulses whilst ch.1 to ch.10 shows the 10 signal output from the game. outputs of the decade counter. You should be able to see that only one output at a time is high and that changes occur on the Segment a: This segment never lights; therefore, the ‘a’ rising edge of the clock pulse. segment can be connected permanently to the 0V line.