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UNIVERSITY OF CALIFORNIA
College of Engineering Electrical Engineering and Computer Sciences Department
145M Microcomputer Interfacing Lab
Final Exam Solutions May 20, 1993
1A R-2R Ladder^ (of a D/A converter):^ Resistor network consisting only of resistors of value R
and 2R that permits N switches to control a binary sequence of currents Ii = I 0 2 i.
1B Average Step Size^ (of an A/D converter):^ Average difference between all neighboring transi-
tion voltages. Also, the last transition voltage V(2N^ –2, 2N^ –1) minus the first transition voltage V(0,1), divided by 2N^ –2.
1C Relative Accuracy^ (of an A/D converter):^ Agreement between transition voltages V(n, n+1)
plotted as a function of n and a straight line passing through the first and last transition voltages.
1D Handshaking^ (between a microcomputer and a digital circuit):^ Signal lines and protocol for
ensuring accurate data transmission. Typical signals are “ready to send”, “ready to receive”, “data sent”, “data received”.
1E Two’s Complement Operation:^ Complement each bit and add 1.
1F Tri-State Buffer:^ Digital circuit with input and output lines, and a select line. When the select
line is asserted, output = input. Otherwise the output is in a high-impedance state.
2A 110V (rms)
156V (p-p) 59-61 Hz
20R
R
Micro- computer
≈8V p-p
Timer/ Counter
8-bit parallel port (not used)
Comparator
2B^1 Set counter 1 for external input- increment on rising edge.
2 Set counters 2 and 3 for cascaded upward counting at 1 MHz 3 Zero and arm all three counters 4 Latch and read counter 1 in a loop until it reads 1 5 Latch and read counters 2 and 3, pack bytes to produce a time T 6 Latch and read counter 1 in a loop until it reads 61 7 Latch and read counters 2 and 3, pack bytes to produce a time T 8 Compute power line frequency as 60 x 10^6 /(T2 – T1) [4 points off if the exact time of zero crossings not used. Counting the number of cycles in 1 sec gives only integer answers 59, 60, and 61- this is not accurate to 0.001 Hz.
2C
59-61 Hz signal
Comparator output
Counter 1 (^0 1 2 )
3A
LPF
LPF
S/H
S/H
A/D
A/D
Tri-state #
Tri-state #
16-bit parallel input
Timer
Hold mode, start conversion
End conversion, latch tri-state parallel output
End conversion, latch tri-state
Select #
Select #
BIHOLD
BISTROBE
3B^1 Initialize timer for positive external pulses, 15 μs wide, 50 kHz rep rate
2 Arm timer 3 Timer pulse puts both S/H in hold mode, starts both A/D converters 4 When conversion is complete, the two A/D converters strobe their data onto their respec- tive tri-state registers. 5 The next timer pulse is detected by the program, which has been looping to sense BISTROBE 6 Program puts out a pulse in output line #1 to select tri-state buffer #1, which asserts its data onto the 16-bit parallel input port 7 After a short delay to allow the data to settle, the program puts a pulse on output line #3, which is connected to BIHOLD and latches the data onto the 16-bit parallel input port 8 The program reads the input port 9 The program puts a pulse on output line #2 to select tri-state buffer #2, which asserts its data onto the 16-bit parallel input port 10 repeat steps 7 and 8 11 Repeat steps 3-4 and 5-10 which occur in parallel, so that the program is reading old data from the tri-state buffers during the 15 μs that it takes the A/D converters to convert new data. Steps 3-4 take 15 μs while steps 5-10 should take about 6 μs.
Method 2 (continuous mode); 1 Play the compact disk data through a standard compact disk player, which interpolates the steps in the D/A output waveform and low-pass filters to give a smooth analog waveform 2 Sample this waveform at 48 kHz and digitize [5 points off if a no interpolation or filtering used to smooth the higher harmonics caused by the abrupt edges of the 44 kHz D/A output steps. These higher frequencies can cause severe aliasing]
[15 points off if the DAT digital values are the same as the CD digital values]
145M Numerical Grades:
5/9 x Lab Lab Partic. Midterm #1 Midterm #2 Final Total Average 478 100 84 92 162 921 (B+) rms 23 0 12 4 28 53 Maximum 500 100 100 100 200 1000
145M Letter Grade Distribution
Letter Grade Course Totals (1000 max)
A+ 990 A 955, 958, 973 A– 935, 941, 943, 946, 946
B+ 919, 921 B 868, 880, 886, 897 B– none
C 772
