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Problems

Exercise

Baud rate is defined as the number of symbols per second transmitted. When using RS-232, this is equivalent to the number of bps. How long with it take to transmit the ASCII string

Real Time Computing!

with 1 parity bit and using RS-232 at 9600 baud?

Exercise

Consider a microprocessor DI with an internal pullup resistor \(R_U\) to \(V_\text{dd}\), as shown in figure 3.5e. We would like to reverse the direction of the floating value of the DI from high (\(V_\text{dd}\)) to low.

  1. Adding an external pulldown resistor \(R_D\) to ground, find the floating input pin voltage \(V_I\) in terms of \(V_\text{dd}\), \(R_U\), and \(R_D\).
  2. Show that for the pin to float in the correct direction, \(R_D < R_U\).
  3. Determine the power \(\mathcal{P}\) required of the digital source to maintain \(V_I\) when the input is floating.
  4. With \(R_U = 40\) k\(\Omega\) and \(V_\text{dd} = 3.3\) V, find the external pulldown resistance \(R_D\) in terms of \(V_I\) and \(\mathcal{P}\).
  5. If our logic family is \(3.3\)-V CMOS, what are the minimum and maximum values of \(R_D\)?
  6. Using your results from the previous parts, plot the required power \(\mathcal{P}\) over the range of \(R_D\) that satisfies our logic family.
  7. Make a recommendation for the value of \(R_D\) for a grid-powered application and another recommendation for a battery-powered application.

Exercise

Consider a microprocessor DI with an internal pullup resistor \(R_U\) to \(V_\text{dd}\), as shown in figure 3.5e. Due to the requirements of an attached digital circuit, we would like to reduce the pullup resistance to \(\gamma R_U\), where \(\gamma \in [0,1]\). We can achieve this by adding an external pullup resistor \(R_{UE}\).

  1. Derive an expression for \(R_{UE}\) in terms of \(\gamma\) and \(R_U\).
  2. If we need \(\gamma = 1/20\), and \(R_U = 40\) k\(\Omega\), which should we choose for \(R_{UE}\)?
  3. If the attached circuit is also floating (high-\(Z\) with no pull resistors), what is the total power \(\mathcal{P}_f\) flowing through the pullup resistors in terms of \(R_U\), \(R_{UE}\), and \(V_\text{dd}\)?
  4. If the attached circuit is a short to digital ground, what is the total power \(\mathcal{P}_s\) flowing through the pullup resistors in terms of \(R_U\), \(R_{UE}\), and \(V_\text{dd}\)?