Lab 5 and 6 notes.

labnotes/Lab 5 and 6.tex

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+\documentclass[letterpaper]{article}
+\author{Jonathan Chan (15354146)}
+\title{PHYS 319\\Labs 5 and 6 Notes}
+
+\usepackage{fullpage}
+\usepackage{minted}
+\usepackage{siunitx}
+\usepackage{hyperref}
+
+\begin{document}
+	\maketitle
+	
+	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:
+		\begin{enumerate}
+			\item Send a trigger pulse to the ranger. According to the documentation, this pulse needs to be $\SI{10}{\micro s}$ long.
+			\item Wait for the echo input to be raised. This can be done by setting a low-to-high interrupt on the echo pin.
+			\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. 
+			\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.
+			\item Send the data to the Python plot. This portion of the code will come from the provided temperature measurement program.
+			\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.
+		\end{enumerate}
+	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 $\SI{2}{cm}$.
+	
+	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}.
+	
+	\newpage	
+	\noindent \textit{\texttt{main.c}}:
+	\begin{minted}{c}
+    #include "msp430.h"
+    #define   TXD       BIT2
+    #define   RXD       BIT1
+    #define   TRIG      BIT4
+    #define   ECHO      BIT6
+    #define   CM        58
+    
+    unsigned int TXByte;
+    void main(void) {
+        WDTCTL    = WDTPW + WDTHOLD;  // Stop WDT
+    
+        BCSCTL1   = CALBC1_1MHZ;      // Set range
+        DCOCTL    = CALDCO_1MHZ;
+        BCSCTL2  &= ~(DIVS_3);        // SMCLK = DCO = 1 MHz
+        P1SEL     = BIT1 + BIT2;      // P1.1 = RXD, P1.2=TXD
+        P1SEL2    = BIT1 + BIT2;      // P1.1 = RXD, P1.2=TXD
+        UCA0CTL1 |= UCSSEL_2;         // Use SMCLK
+        UCA0BR0   = 104;              // Set baud rate to 9600 with 1MHz clock (Data Sheet 15.3.13)
+        UCA0BR1   = 0;                // Set baud rate to 9600 with 1MHz clock
+        UCA0MCTL  = UCBRS0;           // Modulation UCBRSx = 1
+        UCA0CTL1 &= ~UCSWRST;         // Initialize USCI state machine
+    
+        P1DIR    |= TXD;
+        P1OUT    |= TXD;
+        P1DIR    |=  TRIG;            // set trigger output
+        P1DIR    &= ~ECHO;            // set echo input
+        P1IE     |=  ECHO;            // use echo input as interrupt
+        TACTL     = TACLR;            // reset clock
+        TACTL     = TASSEL_2 | MC_2;  // set SMCLK timer to count up at 1 MHz
+        __enable_interrupt();
+    
+        while (1) {
+            P1OUT |=  TRIG;                     // start trigger signal
+            __delay_cycles(10);                 // we need a >10 us pulse and one clock cycle is 1 us
+            P1OUT &= ~TRIG;                     // end trigger signal
+    
+            P1IES &= ~ECHO;                     // interrupt on low to high
+            __bis_SR_register(LPM0_bits + GIE);
+            TAR = 0;
+            P1IES |=  ECHO;                     // interrupt on high to low
+            __bis_SR_register(LPM0_bits + GIE);
+    
+            TXByte = TAR / CM;                  // distance in cm
+            TXByte = (TXByte <= 0xFF) * TXByte; // set to 0 if beyond range
+            while (!(IFG2 & UCA0TXIFG));        // wait for TX buffer to be ready for new data
+            UCA0TXBUF = TXByte;           
+    
+            __delay_cycles(100000);             // wait >10 ms before measuring again
+        }
+    }
+    
+    // handle P1.3 interrupts
+    #if defined(__TI_COMPILER_VERSION__)
+    #pragma vector=PORT1_VECTOR
+    __interrupt void port1_isr(void)
+    #else
+        void __attribute__ ((interrupt(PORT1_VECTOR))) port1_isr (void)
+    #endif
+    {
+        P1IFG = 0;                              // reset interrupt flag
+        __bic_SR_register_on_exit(LPM0_bits);   // take us out of low power mode
+    }
+	\end{minted}
+	
+	\noindent \textit{\texttt{serial-python-plot.py}}:
+	\begin{minted}{python}
+	...
+	ax = fig.add_subplot(111,xlabel='Time Step',ylabel='Distance (cm)')
+	ax.set_ylim(0,255) # set limits of y axis.
+	...
+	\end{minted}
+\end{document}