Lab 5 and 6 notes.
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\documentclass[letterpaper]{article}
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\author{Jonathan Chan (15354146)}
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\title{PHYS 319\\Labs 5 and 6 Notes}
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\usepackage{fullpage}
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\usepackage{minted}
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\usepackage{siunitx}
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\usepackage{hyperref}
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\begin{document}
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\maketitle
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The goal of this lab is to measure distances with the ultrasonic ranger. References to specific values and other documentation notes refer to \url{http://www.robot-electronics.co.uk/htm/srf04tech.htm}. The steps to achieve this can be outlined as follows:
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\begin{enumerate}
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\item Send a trigger pulse to the ranger. According to the documentation, this pulse needs to be $\SI{10}{\micro s}$ long.
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\item Wait for the echo input to be raised. This can be done by setting a low-to-high interrupt on the echo pin.
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\item Time the length of the echo pulse sent by the ranger. We will set \texttt{TAR} to 0 and retrieve the value of \texttt{TAR} when the echo pin is lowered, which can be done by setting a high-to-low interrupt on the echo pin. We will use the SMCLK timer, which runs at $\SI{1}{M\hertz}$, meaning that the maximum length that can be measured without overflowing is $\SI{65 535}{\micro s}$. However, according to the documentation, the ranger will time out and lower the echo pin after $\SI{30 000}{\micro s}$, so we aren't in danger of overflowing.
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\item Convert the measurement to distance in centimetres. According to the documentation, we need to divide by $58$. Since we're limited to sending one byte at a time, we can also set the value to $0$ if the distance exceeds $\SI{255}{cm}$ to prevent the plot from graphing meaningless overflowed values.
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\item Send the data to the Python plot. This portion of the code will come from the provided temperature measurement program.
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\item Repeat the measurement. According to the documentation, we need to wait $\SI{10}{ms}$ to allow the ranger to recharge. I've set the delay to $\SI{100}{ms}$ to smooth out the data and prevent rapid changes in the position of the ranger from being recorded.
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\end{enumerate}
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With some measurements using a metre stick, the distances measured by the ranger are indeed accurate to the centimetre. It also appears that it cannot measure anything closer than $\SI{3}{cm}$. Of course, we are also upper-bounded at $\SI{255}{cm}$ by the size of one byte, even if the limit of the ranger itself is $\SI{30 000}{\micro s} / \SI{58}{\micro s*cm^{-1}} \approx \SI{517}{cm}$. We can circumvent this by dividing the the value by $2$ in the C program then multiplying it by $2$ again in the Python program, so that the maximum measurable distance is $\SI{510}{cm}$, at the cost of decreasing the precision to a granularity of $\SI{2}{cm}$.
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Below is the modified \texttt{main.c} file, with the remaining files unchanged from \url{http://www.phas.ubc.ca/~michal/phys319/temperature_demo4.zip}. Of course, the axis ranges and labels of the plot should be adjusted accordingly. After the C program has been written to the microprocessor, connect P1.4 to the ranger's trigger pin, connect P1.6 to the ranger's echo pin through a $\SI{1}{k\ohm}$ resistor, connect the ranger to $\SI{5}{V}$ and ground, connect the microprocessor to ground, and run \texttt{python python-serial-plot.py}.
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\newpage
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\noindent \textit{\texttt{main.c}}:
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\begin{minted}{c}
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#include "msp430.h"
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#define TXD BIT2
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#define RXD BIT1
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#define TRIG BIT4
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#define ECHO BIT6
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#define CM 58
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unsigned int TXByte;
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void main(void) {
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WDTCTL = WDTPW + WDTHOLD; // Stop WDT
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BCSCTL1 = CALBC1_1MHZ; // Set range
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DCOCTL = CALDCO_1MHZ;
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BCSCTL2 &= ~(DIVS_3); // SMCLK = DCO = 1 MHz
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P1SEL = BIT1 + BIT2; // P1.1 = RXD, P1.2=TXD
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P1SEL2 = BIT1 + BIT2; // P1.1 = RXD, P1.2=TXD
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UCA0CTL1 |= UCSSEL_2; // Use SMCLK
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UCA0BR0 = 104; // Set baud rate to 9600 with 1MHz clock (Data Sheet 15.3.13)
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UCA0BR1 = 0; // Set baud rate to 9600 with 1MHz clock
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UCA0MCTL = UCBRS0; // Modulation UCBRSx = 1
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UCA0CTL1 &= ~UCSWRST; // Initialize USCI state machine
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P1DIR |= TXD;
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P1OUT |= TXD;
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P1DIR |= TRIG; // set trigger output
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P1DIR &= ~ECHO; // set echo input
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P1IE |= ECHO; // use echo input as interrupt
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TACTL = TACLR; // reset clock
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TACTL = TASSEL_2 | MC_2; // set SMCLK timer to count up at 1 MHz
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__enable_interrupt();
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while (1) {
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P1OUT |= TRIG; // start trigger signal
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__delay_cycles(10); // we need a >10 us pulse and one clock cycle is 1 us
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P1OUT &= ~TRIG; // end trigger signal
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P1IES &= ~ECHO; // interrupt on low to high
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__bis_SR_register(LPM0_bits + GIE);
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TAR = 0;
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P1IES |= ECHO; // interrupt on high to low
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__bis_SR_register(LPM0_bits + GIE);
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TXByte = TAR / CM; // distance in cm
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TXByte = (TXByte <= 0xFF) * TXByte; // set to 0 if beyond range
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while (!(IFG2 & UCA0TXIFG)); // wait for TX buffer to be ready for new data
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UCA0TXBUF = TXByte;
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__delay_cycles(100000); // wait >10 ms before measuring again
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}
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}
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// handle P1.3 interrupts
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#if defined(__TI_COMPILER_VERSION__)
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#pragma vector=PORT1_VECTOR
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__interrupt void port1_isr(void)
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#else
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void __attribute__ ((interrupt(PORT1_VECTOR))) port1_isr (void)
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#endif
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{
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P1IFG = 0; // reset interrupt flag
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__bic_SR_register_on_exit(LPM0_bits); // take us out of low power mode
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}
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\end{minted}
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\noindent \textit{\texttt{serial-python-plot.py}}:
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\begin{minted}{python}
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...
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ax = fig.add_subplot(111,xlabel='Time Step',ylabel='Distance (cm)')
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ax.set_ylim(0,255) # set limits of y axis.
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...
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\end{minted}
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\end{document}
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