EXAMINATIONS OF DECEMBER 2003

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

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

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

Question 1:

(a) Sketch the waveforms for the following serial transmissions using 8-bit data,

even parity, one stop bit, and a 56.6K baud rate. Indicate the bit time.

ASCII character (54H) & ASCII character (55H) [6]

(b) Using Figure 1, write a short 8085 assembly language program (mnemonics

and comments) that will read Ports 21H and 23H of the 8155 IC

(on the SDK-85 kit), form a 14-bit number (Port 21H as the LSB), and then

load this number into the Timer of the same 8155 IC used. [6]

(c) Now, write another program that starts the Timer to produce square wave

pulses to flash an LED connected to the Timer Out pin (Sketch your circuit).

At what rate will the LED flash ? [8]

Question 2:

Explain how the 8085 CPU instruction RST n operates. [5]

Assume that an Interrupt Service Routine (ISR) for interrupt TRAP is stored in

memory beginning at 2060H. Explain (and illustrate) what sequence of steps

must occur for this ISR to be activated. [7]

While the ISR above is executing, an RST 7.5 interrupt occurs. Explain, with suitable assembly code, how you would check to determine if RST 7.5 is pending and, if it

is pending, enable it without affecting any other interrupts; otherwise, return to the

main program. [8]

Question 3:

Select any port(s) of the SDK-85 kit and then write an 8085 assembly language

program that will control the 8-bit A/D converter as shown in Figure 2 (below).

Your program should use the EOC pin (in any mode your prefer), read ten (10)

values from the A/D, and place the A/D values in memory from location 2070H. [12]

The sensor shown has a range of 0-1 volt which corresponds to a temperature

of 0-100 °C, comment on the resolution and accuracy of the system. [4]

The conversion time of the A/D is 10ms. Using your program above, calculate

(approximately) the time to read and store ten (10) temperature readings. [4]

Figure 2:

Question 4:

Explain how the Stack operates in a typical 8085 CPU microprocessor system,

using the instructions, PUSH & POP, to illustrate your answer. [6]

Describe, in detail, the process that takes place when the 8085 microprocessor

implements a subroutine. (You should explain the part played by the Stack and

any other registers). [6]

Write a short 8085 assembly language program (mnemonics and comments) that

will save the D-register on the Stack, if both Bit 0 & Bit 1 (of Reg.D) are Set,

Set them if both are zero, otherwise, leave them alone. [8]

Question 5:

Describe the nature and purpose of the main control signals that are used for

accessing both, memory and I/O space, of the Intel 8085 CPU. [8]

Construct a decoded memory table showing how the various outputs

(Y₀ - Y₇) are selected in the 3:8 Address Decoder arrangement of Figure 3.

[Assume A10 - A15 are from the address bus of an 8085 CPU]. [12]

Figure 3

(N.B: in Exam, Data sheets are provided)