EXAMINATIONS OF DECEMBER 2002

Code and Name of Course: EL21C – MICROPROCESSORS I Duration: 2 hours

INSTRUCTIONS TO CANDIDATES: This paper has pages and 5 questions.

Answer any THREE (3) of FIVE (5) questions.

Question 1:

Explain the various Flags available on the 8085 CPU. [6]

A Switch is connected to Bit 0, Port 21H of the 8155 PIA on the SDK-85 microprocessor kit in the lab. Explain and write commented Assembly Language code to read the Switch and turn-on a Buzzer, connected to Bit 7 of Port 22H, if the Switch is on. Show a circuit diagram of the setup and state any assumptions made. [7]

If the Switch were connected to pin RST 5.5 of the CPU instead of to Bit 0, Port 21H, explain (and show code) how you would turn-on the Buzzer. [7]

Question 2:

Design and explain a unique I/O interface to decode port address 3FH on an 8085 system, for both the OUT and IN statements respectively. [7]

Figure 1 shows the main address decoder for an 8085 CPU based computer. Determine the address ranges of the 3:8 decoder. [6]

Explain how would you interface another 3:8 decoder to double the address range. Show your schematic and the address ranges added. [7]

Question 3:

Sketch the asynchronous serial waveform of the ASCII character M (4FH), using 8 data bits, one stop bit, odd parity, and a baud rate of 56.6 kbps. [5]

Write a routine that would send out a character on the SOD pin of an 8085 CPU, using the specification above (ignore baud rate and assume character in the C-register). [10]

Discuss the advantages/disadvantages of the method of serial communications used by the 8085 CPU and indicate how newer CPU accomplish this task. [5]

Question 4:

The A/D converter in Figure 2 is to be interfaced to your SDK-85 kit. Show and explain your schematic. Assume the A/D converter must be interrupt controlled using RST5.5. You may use any port(s) on the kit. [10]

Write the subroutine driver that handles the A/D converter, placing any data read in memory location 20F0H. [10]

Question 5:

( a ) Carefully explain how the Stack operates on the 8085 CPU. [6]

( b ) The Clock-In pin of the 8155 PIA on the SDK-85 kit in the lab is connected to a 1 MHz

source. Explain, with code, how you could obtain a pulse, every 100 ms. [6]

( c ) Carefully explain the stages involved in executing a subroutine CALL and RET

instructions. Use diagrams to illustrate your answer. [8]