Transcript Microprocessors I

16.317
Microprocessor Systems Design I
Instructor: Dr. Michael Geiger
Fall 2014
Lecture 31:
PIC exam practice problems
Lecture outline
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Announcements/reminders
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HW 5 due Monday, 12/1
HW 6 due Wednesday, 12/10
No more lectures of new material
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Exam 3: Monday, 12/15, 3:00-6:00 PM
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Next lecture: Exam 3 Preview, Wednesday, 12/10
I’ll be in my office at 8:00 every MWF until classes end
(with the exception of Wednesday, 11/26)
Will need to complete course eval (to be posted online)
prior to exam
Today’s lecture: Exam practice problems
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Review: Analog to digital conversion
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10-bit resolution (result = (V / VREF) * 1023)
Configuration requires
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Choosing reference voltage
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Choosing conversion speed via clock source
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Appropriate bit(s) = 1 in ANSELx register
Left- or right-justifying result
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11 channels split across ports A, B, C
Configuring input channel(s) as analog input(s)
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Divided system clock or external oscillator
Choosing input channel
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VDD/VSS, internal fixed voltage ref, or external source
L: ADRESH = upper 8 result bits, lower 6 bits of ADRESL = 0
R: ADRESL = lower 8 result bits, upper 6 bits of ADRESH = 0
Setting ADON bit in ADCON0
Conversions
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Wait ~5 us for charging cap to settle
Assert GO bit in ADCON0
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Bit will be cleared when conversion is complete
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Exam practice problems
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Two problems from Spring 2014 exam
Given short, partially complete programs to
be completed
Comments next to each blank tells you what
statement(s) in those spaces should do
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Example 1 solution
void interrupt ISR(void) {
if (IOCAF) {
IOCAF = 0;
__delay_ms(5);
if (SWITCH == DOWN) {
LATC = 0;
i = 0;
}
}
if (INTCONbits.T0IF) {
INTCONbits.T0IF = 0;
LATC = st[i];
//
//
//
//
//
//
SW1 was pressed
Clear flag in software
Delay for debouncing
If switch still pressed
clear LEDs
and reset i
i++;
//
//
//
//
//
Timer 0 interrupt
Clear flag in software
Update LEDs to show
current st[] value
Increment i
if (i > 7)
i = 0;
// If i exceeds max index
//
reset i
}
}
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Example 2 solution
void read_adc(void) {
unsigned char lobits;
//
//
//
//
//
__delay_us(5);
GO = 1;
while (GO) continue;
lobits = ADRESL & 0x03;
if (lobits == 0b01) {
LATC = LATC ^ 0x01;
}
else if (lobits == 0b10) {
LATC = LATC ^ 0x02;
}
else if (lobits == 0b11) {
LATC = LATC | 0x03;
}
Variable to hold lowest 2
bits of ADC result
Wait for ADC cap to settle
Start conversion
Wait until conversion done
// lobits = lowest two bits
//
of ADC result
// In this case, toggle
//
lowest LED
// In this case, toggle
//
second LED
// In this case, turn first &
//
second LEDs on
}
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HW 6 notes
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Will write three programs
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Display sequence of values on LEDs, changing
once per second
Count button presses and display count on LEDs
Variable speed/direction LED rotation
Specification also contains helpful hints for
setting up and completing programs
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Final notes
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Next time:
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Exam 3 Preview (Wednesday, 12/10)
Reminders:
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HW 5 due Monday, 12/1
HW 6 due Wednesday, 12/10
No more lectures of new material


Exam 3: Monday, 12/15, 3:00-6:00 PM

7/18/2015
I’ll be in my office at 8:00 every MWF until classes end (with
the exception of Wednesday, 11/26)
Will need to complete course eval (to be posted online) prior
to exam
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