E. T. TELECOMUNICACIONS 1BM2 DIGITAL ELECTRONICS 2/06/2008 Prof. F. J. Sànchez i Robert - Seventh minimum control: 45 min. Grades will be available on June 09th - Questions about the examination: TH:17 h – 19 h; FR: 10 h-14h VERY IMPORTANT: Draw a general schematic or plan, develop the exercise and justify the results always explaining what are you doing Minimum 7: Designing and working with counters and registers In this exercise our aim is to design a digital programmable PWM. Our intention is to apply our device to control the speed of a DC motor or the luminosity of a column of LED (like a modern dimmer, see this reference1). Fig. 1 shows the entity for the proposed circuit. In this simple example, let us to consider a ladder of 16 levels of duty cycle (DC), thus only 4-bits will be required. From the code “0000” that will give a 0% DC, to the code “1111” that will represent a 100% of DC. The output PWM waveform will have 1/16 of the input clock frequency. UP Level of DC PWM_GEN R8 UP DOWN DOWN a_L b_L c_L d_L e_L f_L g_L Vcc 330 OSC R1 CLK PWM_OUT LED CCT001 CLOCK=16kHz Fig. 1 Block diagram for the programmable PWM and typical waveforms 1. Invent the internal design for the PWM_GEN using counters and comparators and explain the mode of operation by means of a time diagram. Pushbuttons must work at 1 Hz. NOTE: This point will be discussed in class 2. If a pair of universal counters (Fig. 2) and a standard 4-bit comparator are used, design the extra interface logic so that the device can be commanded by the UP and DOWN pushbuttons. Make also the PWM output synchronous with the input clock. 3. Design the 4-bit comparator. 4. Design the synchronous counter block shown in Fig. 2, with Up-Down (UD_L), Count Enable (CE), and also a synchronous Set Direct (SD) signals. a. Plan a general structure for the counter following the standard FSM architecture and using multiplexers to establish the signals precedence. 1 http://www.reuk.co.uk/LED-Dimmer-Circuit.htm b. Produce the SC1 internal design for the Up, Down, and count disable modes of operation. 5. Verify your design using Proteus once you had worked all the subcircuits. CE UD_L Function Q[3..0] 0 X Inhibit TC 1 1 Up count 1 0 Down count COUNTER CE UD_L CLK SD 4-BIT_COUNTER Fig. 2 Entity for the 4-bit synchronous counter to be designed