Difference between revisions of "Embedded System Tutorial UART"

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<font color="red"> This article is under construction</font>
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Socialledge is moving to two portals. 
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*  The Wiki will remain here for general references about the SJ-One board, and to document student reports.
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*  The bookstack will now be used for SJSU assignments
  
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[http://books.socialledge.com/books/embedded-drivers-real-time-operating-systems/chapter/lesson-uart This article has been moved here]
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== Introduction ==
 
== Introduction ==
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'''UART''' stands for '''U'''niversal '''A'''synchronous '''R'''eceiver '''T'''ransmitter.  There is one wire for transmitting data, an done wire to receive data.  A common parameter is the baud rate known as "bps" which stands for '''b'''its '''p'''er '''s'''econd.  If a transmitter is configured with 9600bps, then the receiver must be listening on the other end at the same speed.
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The objective of this lesson is to understand UART, and use two boards and setup UART communication between them.
  
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UART is a serial communication, so bits must travel on a single wire.  If you wish to send a '''char''' over UART, the char is enclosed within a '''start''' and a '''stop''' bit, so to send 8-bits of '''char''' data, it would require 2-bit overhead; this 10-bit of information is called a '''UART frame'''.  Let's take a look at how the character 'A' is sent over UART.  In ASCII table, the character 'A' has the value of 65, which in binary is: 0100.0101 If you inform your UART hardware that you wish to send this data at 9600bps, here is how the frame would appear on an oscilloscope :
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'''UART''' stands for '''U'''niversal '''A'''synchronous '''R'''eceiver '''T'''ransmitter.  There is one wire for transmitting data, and one wire to receive data.  A common parameter is the baud rate known as "bps" which stands for '''b'''its '''p'''er '''s'''econd.  If a transmitter is configured with 9600bps, then the receiver must be listening on the other end at the same speed.
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<Picture needed>
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UART is a serial communication, so bits must travel on a single wire.  If you wish to send a '''char''' over UART, the char is enclosed within a '''start''' and a '''stop''' bit, so to send 8-bits of '''char''' data, it would require 2-bit overhead; this 10-bit of information is called a '''UART frame'''.  Let's take a look at how the character 'A' is sent over UART.  In ASCII table, the character 'A' has the value of 65, which in binary is: 0100-0001.  If you inform your UART hardware that you wish to send this data at 9600bps, here is how the frame would appear on an oscilloscope :
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[[File:uart_tutorial_frame.jpg|center|frame|UART Frame of 'A']]
  
 
A micrcontroller can have multiple UARTs in its hardware, and usually UART0 is interfaced to a "USB to serial" converter chip which is then connected to your computer.  In this exercise, you will write a driver for UART-2 and attempt to communicate between two boards.
 
A micrcontroller can have multiple UARTs in its hardware, and usually UART0 is interfaced to a "USB to serial" converter chip which is then connected to your computer.  In this exercise, you will write a driver for UART-2 and attempt to communicate between two boards.
  
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I encourage you to fully read this article first, and here is a video about the UART0 tutorial.  This is a FAST PACED video, so learn to pause the video and look over your LPC user manual frequently :)  '''Note that I forgot to configure the PINSEL registers, which are covered by this tutorial below.'''
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*  [https://www.youtube.com/watch?v=RU_NUPprx2Y UART Driver Video]
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<BR/>
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== UART Pins ==
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Before you write a UART software driver, you need to understand the physical constraints and identify the UART pins on your processor.  A GPIO (general purpose input output) is a multi-purpose pin, and certain pins are used for certain functions.  For example, P0.0 and P0.1 on your LPC17xx processor can be used for an LED (output), a switch (input), or as UART transmitter and receive signals.  We will configure the microcontroller's internal MUX (multiplexor) to connect internal UART to external pins.
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[[File:tutorial_uart_pinsel.png|center|Find RXD2 and TXD2 of UART2]]
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<BR/>
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[[File:tutorial_gpio_mux.png|center|Example MUX that we need to configure for a PIN selection]]
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<BR/>
  
 
== Clock System & Timing ==
 
== Clock System & Timing ==
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9600bps means that one bit takes 1/9600 = 104uS (micro-seconds) per bit.  The idea is that we want to divide the peripheral clock to UART hardware by a number that yields roughly 104uS per bit.  The '''Software Driver''' section goes over how to configure your UART driver to divide the clock to yield the desired baud rate.
 
9600bps means that one bit takes 1/9600 = 104uS (micro-seconds) per bit.  The idea is that we want to divide the peripheral clock to UART hardware by a number that yields roughly 104uS per bit.  The '''Software Driver''' section goes over how to configure your UART driver to divide the clock to yield the desired baud rate.
  
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[[File:uart_tutorial_clock.jpg|center|frame|Example clock system of LPC17xx]]
  
 
== Hardware Design ==
 
== Hardware Design ==
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== Software Driver ==
 
== Software Driver ==
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The UART chapter on LPC17xx has a really good summary page on how to write a UART driver :
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[[File:uart_tutorial_overview.jpg|center|frame|UART chapter summary]]
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{|
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|
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The UART chapter on LPC17xx has a really good summary page on how to write a UART driver.  '''Read the register description of each UART register to understand how to write a driver.'''  This tutorial gives away answers but unless you spend 1-2 hours reading the UART chapter, you will forget this knowledge.  The datasheet shows many registers but remember that for a simple driver, we will not need interrupts so you can skip the sections that talk about the UART interrupts.
  
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[[File:uart_tutorial_overview.jpg|center|frame|UART chapter summary]]
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<BR/>
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Writing a uart initialization routine is simple except that some registers require a special setup to access them.  In the sample code below, we power up UART0, and use the PINSEL register to configure the Rx/Tx Uart pins as the pin functionality.  Finally, we just set the divider to achieve 9600bps.  The exception here is that we have to read the datasheet carefully which states that the '''DLM''' and the '''DLL''' registers are only accessible if the '''DLAB''' bit is set at the '''LCR''' register.
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Notice that four registers have the same address.  The UART divider registers are only accessible if DLAB bit is 1; this was done to protect accidental change of baud rate.  Furthermore, notice that the CPU is intelligent enough to know if you are accessing the RX or the TX register based on if the register is being read or written.
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<syntaxhighlight lang="c">
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void uart0_init(void)
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{
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    LPC_SC->PCONP |= (1 << 3);      // Enable power to UART0
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    LPC_SC->PCLKSEL0 &= ~(3 << 6);
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    LPC_SC->PCLKSEL0 |=  (1 << 6);  // Uart clock = CPU / 1
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    LPC_PINCON->PINSEL0 &= ~(0xF << 4); // Clear values
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    LPC_PINCON->PINSEL0 |= (0x5 << 4);  // Set values for UART0 Rx/Tx
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    LPC_UART0->LCR = (1 << 7); // Enable DLAB
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    /* See the formula picture to get more info.
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    * Default values of fractional dividers simplifies the equation
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    * Warning: You need to set DLM/DLL correctly, but if divider is small enough, it will fit inside DLL
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    */
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    LPC_UART0->DLM = 0;
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    LPC_UART0->DLL = CPU_CLOCK / (16 * 9600) + 0.5);
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    LPC_UART0->LCR = 3;        // 8-bit data
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}
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</syntaxhighlight>
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| [[File:uart_tutorial_dlab.jpg|center|frame|DLAB bit to access registers]]
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  [[File:uart_tutorial_formula.jpg|center|frame|Baud rate formula]]
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|}
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<syntaxhighlight lang="c">
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char uart0_putchar(char out)
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{
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    LPC_UART0->THR = out;
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    while(! (LPC_UART0->LSR & (1 << 6)));
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    return 1;
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}
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</syntaxhighlight>
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<BR/>
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To send a char over UART, the code looks incredibly easy; just two lines!  It is supposed to be very easy because the UART hardware is supposed to handle the UART frame, and send start bit, followed by 8-data bits, and a stop bit by simply writing the '''THR''' register.  The moment you write this register, the hardware will start shifting bits out of the wire.  The while loop is present because after you write the '''THR''' register, we want to wait until hardware is done sending the bits out of the wire otherwise writing the same register again will corrupt the on-going transfer.
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<BR/>
  
 
== Advanced Design ==
 
== Advanced Design ==
 
What you've done so far is wrote a polling UART driver.  If you used 9600bps, and sent 1000 characters, your processor would basically enter a "busy-wait" loop and spend 1040ms to send 1000 bytes of data.  You can enhance this behavior by allowing your <code>uart_putchar()</code> function to enter data to a queue, and return immediately, and you can use the "THRE" or "Transmitter Holding Register Empty" interrupt indicator to remove your busy-wait loop while you wait for a character to be sent.
 
What you've done so far is wrote a polling UART driver.  If you used 9600bps, and sent 1000 characters, your processor would basically enter a "busy-wait" loop and spend 1040ms to send 1000 bytes of data.  You can enhance this behavior by allowing your <code>uart_putchar()</code> function to enter data to a queue, and return immediately, and you can use the "THRE" or "Transmitter Holding Register Empty" interrupt indicator to remove your busy-wait loop while you wait for a character to be sent.
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== Assignment ==
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* <b>Assignment Outline</b>
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*: Form 2 people teams for this assignment.
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*: Write a driver for UART2 or UART3
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*: Do not use the pre-built driver or Uart2/3 class
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*: Connect your UART to your partner's UART (to his board)
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*: Prove that you can send/receive data across multiple boards.
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*: Upload Logic Analyzer Screenshot for the UART Frame.
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* <b>Extra Credit</b>:
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*: Use Uart interrupt to queue the received data to avoid polling.  You just need to enable UART RX interrupt and then hookup an interrupt callback to do the receive.
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* <b>Steps</b>
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*: Locate the physical pins for a UART that is not already used by your board
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*: Configure the PINSEL to use the pins for UART Rx/Tx
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*: Initialize your UART at any baud rate
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*: Write uart_putchar(char) and uart_getchar() functions
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*: Interface your UART with someone else's board, and test the communication.
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-->

Latest revision as of 20:05, 25 January 2019

Socialledge is moving to two portals.

  • The Wiki will remain here for general references about the SJ-One board, and to document student reports.
  • The bookstack will now be used for SJSU assignments

This article has been moved here


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