Description
Beyond parallel communication, where multiple bits of data are transferred together via a bus of pins[1], serial communication is another method in which a processor can communicate with external devices. Overall, serial communication involves the process of sending one bit of data at a time, while following a specific protocol.
Serial communication is generally classified as either being asynchronous or synchronous. Unlike synchronous serial communication, where a common clock signal is used to determine when to send, receive, or sample data between multiple devices, there exists no synchronization signal for asynchronous serial communication. Instead, asynchronous communication protocols rely on the hope that a common transfer rate will be upheld by any systems using a chosen protocol; if this is not achieved, data transferred or received could be wrongly interpreted, or even entirely missed.
To facilitate asynchronous serial communication, a device known as a Universal Asynchronous Receiver/Transmitter (UART) is generally used in conjunction with both a transmitting device and a receiving device. A UART utilizes a clock signal to generate a transfer rate, denoted as the baud rate[2], and also uses two physical connections to communicate data: one pin to receive data (Rx), and one pin to transmit data (Tx). Within the ATxmega128A1U, several UART modules are available by way of the Universal Synchronous/Asynchronous Receiver Transmitter (USART) system.[3]
LAB STRUCTURE
Within this lab, you will begin to explore the asynchronous capabilities of the USART system within the ATxmega128A1U. In § 1, you will research relevant information regarding the USART system. In § 2, you will learn to transmit a text character from your microcontroller to a connected computer, via the USART system. In § 3, you will use an oscilloscope to measure the transmission signal generated from a relevant USART module. In § 4, you will learn to transmit a character string of arbitrary length4. In § 5, you will determine how to receive a character from a connected computer via the USART system. In § 6, you will learn to receive a character string of arbitrary length4. In § 7, you will configure the reception of data via the USART system to be interrupt-based.
REQUIRED MATERIALS SUPPLEMENTAL MATERIALS
- Atmel ATxmega128A1U AU Manual (doc8331)
- Atmel ATxmega128A1U Manual (doc8385)
- OOTB µPAD v2.0 with USB A/B cable
- Digilent Analog Discovery (DAD) kit with WaveForms software
PRE-LAB PROCEDURE
1. INTRODUCTION TO USART
| PRE-LAB EXERCISES
i. The sampling rate of a UART receiver is usually faster than the baud rate of the overall system. Why is this so? ii. What is the maximum possible baud rate for asynchronous communication within the USART system of the ATxmega128A1U, assuming that the microcontroller has a system clock frequency of 2 MHz and that the USART “double-speed mode” is disabled (i.e., the relevant bit CLK2X is set to 0)? In addition to the maximum rate, provide the values of the relevant registers used to configure that rate. Whenever appropriate, support your answer with calculations. iii. In the context of the USART system within the ATxmega128A1U, how many buffers (i.e., memory locations that store temporary data) are used by a transmitter? How many are used by a receiver? Additionally, for both transmitters and receivers, explain how the use of buffers provides greater flexibility to an application involving these components. iv. If an asynchronous serial communication protocol of 8 data bits, one start bit, one stop bit, no parity, and baud rate of 150 kHz was chosen, calculate how many seconds it would take to transmit the ASCII character string “Dr. Schwartz saw seven slick slimy snakes slowly sliding southward.” (This string has 67 characters.) Show all work. |
| NOTE: It is said that the microcontroller receives data transmitted from the EDBG chip, and that the EDBG receives data transmitted from the microcontroller. Following this nomenclature, recognize that the signal labeled EDBG_USART_CDC_TX in the µPAD schematic is meant to correspond to the Rx signal within the relevant USART module, and the signal labeled EDBG_USART_CDC_RX is meant to correspond to the Tx signal within the same USART module. |
In this section, you will explore documentation for the Universal Synchronous/Asynchronous Receiver Transmitter (USART) system within the ATxmega128A1U, which provides the microcontroller with forms for both synchronous and asynchronous serial communication. However, in this lab, we will only explore the asynchronous form provided by the system.
1.1. Read any relevant parts of § 23 (USART) within the 8331 manual to learn about the asynchronous abilities of the USART system available within the ATxmega128A1U.
On the µPAD, the ATxmega128A1U uses a specific USART module to transfer data, via an EDBG chip, to and from the USB type B port also located on the µPAD; this allows communication between the USB type B port and a USB type A port on a computer, whenever the appropriate USB A/B cable is connected. (Note that the EDBG chip is not shown on the µPAD schematic due to the request of the manufacturer.)
1.2. Review the relevant µPAD schematic to identify which USART module is used to communicate with a connected computer.
2. USART, CHARACTER TRANSMISSION
In this section, you will write an assembly program, lab5_2.asm, to configure the appropriate USART module within your microcontroller to send data to your computer via the relevant USB ports. For this lab, you must utilize the following asynchronous serial communication protocol: odd parity, 8 data bits, 1 start bit, 1 stop bit, and 115,200 bps (bits per second) baud rate.
2.1. Create an assembly program, lab5_2.asm. In this program, first create the following two subroutines.
2.1.1. USART Initialization (USART_INIT). This subroutine should initialize the necessary USART module.
2.1.1.1. Set the data direction of the appropriate USART transmit pin.
2.1.1.2. Configure the USART module for the appropriate mode (synchronous, asynchronous, etc.), and configure the expected number of data bits, parity type, and number of stop bits.
2.1.1.3. Set the baud rate by storing the appropriate value in the relevant baud rate registers. (See Appendix B for
some information regarding baud rates.) You can use the excel workbook given on the course website (Baud Calculator) to verify any baud rate calculations, but be sure that you know how to calculate the necessary values by hand for quizzes and exams. (When configuring baud rate registers, it is useful to use assembler directives when programming in assembly, or macros when programming in C, which will be relevant later.)
2.1.2. Output Character (OUT_CHAR). This subroutine will output a single character to the transmit pin of a chosen USART module. It will be assumed that the relevant character is passed into the subroutine via a general-purpose register (e.g., R16 or R17).
2.1.2.1. At the beginning of this subroutine, check if there is currently an ongoing transmission in the relevant USART module; if there is, wait until it has been completed. An appropriate interrupt flag should be polled to handle this, i.e., do not use an interrupt.
2.1.2.2. Transmit the character passed into the subroutine.
2.2. Next, create a main routine within the assembly program to continually transmit the ASCII character U (i.e., the capital letter U) utilizing the OUT_CHAR subroutine.
You will need to use a serial terminal program on your computer to view any data transmitted by your microcontroller. Basic information on how to configure/use PuTTY, a popular serial terminal program, is given in Appendix A of this document. Some other popular terminal programs are X-CTU, RealTerm, Bray Terminal (also known as Br@y++ Terminal), MobaXterm, and HyperTerminal. There is even a terminal within the Data Visualizer extension of Atmel Studio. You may use any serial terminal program for this course, as long as it has all features needed. (These features will not be listed here.)
| 3. USART, MEASURING BAUD RATE |
2.3. Use a serial terminal program on your computer, e.g., PuTTY, to test your assembly program and verify that the ASCII character U is continually transmitted.
In this section of the lab, you will use your DAD to measure a baud rate created by a USART module, and then record a transmission frame for the ASCII character U.
| NOTE: When utilizing a separate USART module, you will not be able to communicate with a connected computer, since the µPAD was not designed to connect any other module to the relevant EDBG chip. |
Unfortunately, on the µPAD, there is no practical way to measure the physical pins utilized by the USART module connected to your computer. Thus, so that any measurements may be easier, you will utilize some other USART module that has the relevant signals mapped to more accessible pins. routine. (Other than the different USART module, this program should be unchanged from your previous program.)
| transmission frame screenshot, annotate each element within one frame by type, e.g., start bit, data bit 0 (D0), data bit 1 (D1), etc.
3.1. Create an assembly program, lab5_3.asm, to configure NOTE: 0b 01010101) was chosen The ASCII character for the above tasks to allow the U (equivalent to 0x55 = a USART module that has its Tx signal connected to an I/O elements within a character transmission frame to be easily pin that can be easily measured via the µPAD, and then to identified. continually transmit the ASCII character U within a main 4. USART, STRING TRANSMISSION Now that you have a method to output a single character via the 4.1.1.1. Read the character pointed to by Z and increment USART system, you should be able to easily create a routine to the pointer. output a character string of arbitrary length. 4.1.1.2. For each non-null (i.e., non-zero) character, call 4.1. Create an assembly file, lab5_4.asm. First, copy the the subroutine OUT_CHAR; when a null character subroutines used in § 2. Then, write the following is found, return from the subroutine. subroutine. 4.2. Create a main routine within the relevant assembly
4.1.1. Output character string (OUT_STRING). This program to output your complete name, using the subroutine should output a character string stored in OUT_STRING subroutine. Use a terminal program on program memory, using the appropriate USART your computer to test your assembly program. module. When this subroutine is called, it will be assumed that the Z register already points to the beginning of a character string within memory, i.e., any main program utilizing this subroutine must properly configure the Z register before calling the subroutine. 5. USART, CHARACTER INPUT |
3.2. To verify that the defined protocol is met (i.e., 115,200 Hz baud rate, odd parity, 8 data bits, 1 start bit, and 1 stop bit), use the Scope feature within WaveForms, along with your DAD, to measure the width of both a single data bit and a single character transmission frame. Take an appropriate screenshot for each measurement. For the character
In this section, you will begin to configure the appropriate USART module to receive serial data from your computer.
5.1. Create an assembly file, lab5_5.asm. First, copy the subroutines used in § 4, and edit the USART_INIT subroutine to additionally enable the receiver within the appropriate module and configure the data direction of the pin connected to the Rx signal. Then, write the following subroutine.
5.1.1. Input character (IN_CHAR). This subroutine should receive a single character with the relevant USART module and return the received character to the calling procedure via a specified general-purpose register (e.g., R16 or R17).
5.1.1.1. Check if a character has been received (by polling an appropriate interrupt flag), and if not, keep checking until one has been received.
5.1.1.2. Read the received character from the appropriate buffer and return the character to the calling procedure.
5.2. Design a main routine within the relevant assembly file to
| continually echo (i.e., transmit back) to your computer any
6. USART, STRING INPUT
character is found, decrement the Y index to 7. USART, INTERRUPT-BASED RECEIVING
computer any character received. Additionally, to NOTE: Recall that interrupt service routines should generally be demonstrate that your serial interrupt is independent from as short as possible; thus, it would generally be unwise to call a the rest of your program, continually toggle the subroutine within an ISR. |
character received. Utilize the IN_CHAR and OUT_CHAR subroutines whenever appropriate. Use a terminal program on your computer to test your assembly program.
respectively. This sequence of characters originates from typewriter times, when starting a new line of text involved the two-step process of turning a carriage to the position denoting the beginning of a line and then turning the platen (wheel) to the following line.
In systems based on Unix® or Linux®, a newline character consists of only a line feed character.
PRE-LAB PROCEDURE SUMMARY
- Answer pre-lab exercises when applicable.
- Become introduced to the USART system within the ATxmega128A1U in § 1. 3) Learn how to transmit a character via the USART system in § 2.
- Measure USART character transmissions with an oscilloscope in § 3. Take an appropriate screenshot for each relevant measurement and annotate when appropriate.
- Learn to transmit a character string of arbitrary length via the USART system in § 4.
- Create a subroutine to receive a character via the USART system in § 5.
- Implement a subroutine to receive a character string of arbitrary length via the USART system in § 6.
- Create an interrupt-driven echo program in § 7.
[1] You have previously utilized parallel communication whenever using an I/O port or the external bus interface (EBI) system.
[2] To learn more about baud rates in the context of this course, see Appendix B of this document.
[3] In addition to providing the functionality of a UART, the USART system also provides a form of synchronous serial communication to the ATxmega128A1U. 4 By “arbitrary length”, we mean any length of allottable, contiguous data memory locations.
[4] To denote when text should start to appear at the beginning of a following line, a special sequence of values, comprising what is known as a newline, is utilized. Usually, a newline is generated by an Enter key or Return key on a computer keyboard, and, with most operating systems, a newline consists of at least a line feed (LF, 0x0A, \n) character. However, with PuTTY and some other serial terminal programs, only a carriage return (CR, 0x0D, \r) character is used to represent a newline, at least by default.
In the Windows® operating system, a newline is a carriage return character, followed by a line feed character, to designate that a relevant cursor should both return to the beginning of a line of text and advance to the next line of text,
[5] The delete character code is included here since serial terminal programs (e.g., PuTTY) utilize, by default, the delete character code to represent a “backspace”.
