CE121 -- Microprocessor System Design
Assignment 5
Out: February 7, 1997
Due: February 14, 1997

1. (15 points) Problem 8.4 in the text.
2. (20 points) A UART has a shift register and a one-byte character input register. At 300 baud, how long does a microprocessor have to respond to an interrupt indicating a received character, assuming 8 bits, no parity, and one stop bit? Repeat the calculation for 9600 baud, 56 kbaud, and 10 Mbaud. Approximately how many instructions does each of these correspond to for an HC11 with a 4MHz crystal?
3. (15 points) Consider the RS-423 standard.
   1. What is the duration (in seconds) of a bit at the maximum baud rate?
   2. What is the length (in meters) of a bit at the maximum baud rate? Assume that the signal velocity in the cable is m/s.
   3. How many bits can be on the longest possible cable at the maximum baud rate? How many bits can be on the longest suggested cable at the maximum baud rate?
4. (40 points) You have decided to add external vectored interrupts to the HC11. To do this, you implement a daisy-change interrupt circuit and have a specific address (say, ) in the address space you decode to mean ``interrupt acknowledge,'' at which point your device will place a byte of data on the data bus off of which the processor will vector. This is a hardware/software solution. The interrupt is signaled at a hardware level to the HC11, which vectors to the appropriate software routine. The routine must read a value from location . The daisy chain and decoding are used to decide (in hardware) which device should place a vector on the bus. Your software routine should then use this to vector to the correct location.
   1. Design and document circuitry to implement the priority daisy chain. Use and signals. Design only the circuitry for a single device with an input, a input (tied to the decoding), an output. Include a 1-byte register whose value is placed on the databus when is asserted and that device is the highest priority.
   2. Using just a box for each instance of the circuitry above, diagram 3 devices hooked up to the HC11.
   3. Write and document the HC11 assembly code that can make use of the daisy chain hardware to provide vectored interrupts.