Digital circuits require actions to take place at precise times, so they are controlled by a clock that generates a steady sequence of rectangular voltage pulses. One of the most widely used integrated circuits for creating clock pulses is called a 555 timer. FIGURE P28.77 shows how the timer??s output pulses, oscillating between 0 V and 5 V, are controlled with two resistors and a capacitor. The circuit manufacturer tells users that T_H, the time the clock output spends in the high (5 V) state, is T_H= (R₁+ R₂) C × ln 2. Similarly, the time spent in the low (0 V) state is T_L= R₂C × ln 2. You need to design a clock that will oscillate at 10 MHz and will spend 75% of each cycle in the high state. You will be using a 500 pF capacitor. What values do you need to specify for R₁and R₂?

5 V 555 Timer R -5 V Out t OV FIGURE P28.77