NU32: Serial Communication with the PC

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The NU32 has the ability to talk directly with a computer over a virtual serial port connection. This ability can be used to debug code, transfer data, and interact with the microcontroller using a computer.

Overview

The NU32 uses an on board FT2232H chip from FTDI to convert RS-232 TTL serial communication to USB. Using drivers from FTDI, the communication can be opened by any program that can access a virtual serial port. On the NU32, sample code has been created to enable the UART module on the PIC32. Using this code you can write strings to the computer, and generate an interrupt when characters are received. On the computer, several programs that can access the serial connection are discussed, including Processing, MATLAB, and terminal programs.

Details

The FT2232H chip is hardwired the to UART1 (F2 and F8, for general communication) and UART4 (D14 and D15, for bootloading) modules on the PIC32MX795F512L. The Receive (RX) pins, F2 and D14, and Transmit (TX) pins, F8 and D15, are also brought out to the sides of the NU32 board. They should not be used as general IO.

Serial communication is asynchronous, it does not use an external clock signal to know when to read individual bits. Instead, the sender and receiver decide beforehand what frequency to transmit data. This frequency is called the baud rate and must match on both the NU32 and computer for the data to be interpreted correctly.

The NU32 uses the UART1 and UART4 modules at 115200 baud, as does the sample code below.

Library Functions

This code contains functions to read characters and write strings over the serial port. It uses the same header files as "Hello World".

• NU32.c contains the functions to initialize the LEDs and USER button on the NU32 board, and to initialize the UART modules.
• NU32.h contains function prototypes for the .c file.
  • You do not need to modify these files to use serial communication.

The functions that control the serial communication are:

• void initSerialNU32(void) - enables UART1 and UART4 at 115200 baud with an interrupt at priority level 2 and 3
• void WriteString(UART_MODULE, const char *) - call with UART1 and the character array you wish to send
• void PutCharacter(UART_MODULE, const char) - writes an individual character to the computer when the module is able to, called by WriteString()

NU32_serial_example.c in the .zip file above contains the following code that shows how to use the functions. It also contains the ISR for UART1, with an example of how to receive characters.

Sample Code

To initialize the serial communication, call initSerialNU32():

  initSerialNU32();

To write a string to the computer, use WriteString(UART1, charArray)

  WriteString(UART1, "\r\nHello World!\r\n");

To write a string with the value of a variable in it, use sprintf(RS232_Out_Buffer,charArray) and WriteString(UART1, charArray). RS232_Out_Buffer is declared in NU32.h as a char array with 32 elements.

  sprintf(RS232_Out_Buffer, "the value of i is: %d\r\n", i);
  WriteString(UART3, RS232_Out_Buffer);

To receive characters in an interrupt, use this interrupt service routine (ISR):

void __ISR(_UART_1_VECTOR, ipl2) IntUart1Handler(void) {
  // Is this an RX interrupt?
  if(INTGetFlag(INT_SOURCE_UART_RX(UART1))){
    char data = UARTGetDataByte(UART1);

    // now do something with data

    // Clear the RX interrupt Flag
    INTClearFlag(INT_SOURCE_UART_RX(UART1)); 
  }

  // If the interrupt was on TX (ignore it)
  if(INTGetFlag(INT_SOURCE_UART_TX(UART1))) {
    INTClearFlag(INT_SOURCE_UART_TX(UART1));
  }
}

NU32_serial_example.c initializes the serial communication, and then waits until it receives the character 'a' from the computer. When it gets an 'a', it replies back with numbers 0 through 9, and then waits for another 'a'.

Communicating with NU32_Utility

The right hand side of the NU32_Utility program is a debugger that will display any character received from the 'Debug' port, and will send any characters in the text entry using 'Send'.

Communicating with a Terminal Program

The NU32_Utility debugger is based on a more traditional Terminal program. Terminal programs let you choose the baud rate, flow control, and many other aspects of serial communication, and let you view the incoming bytes as ASCII text or hex bytes. A Terminal program is useful when you want to see hidden characters that the NU32_Utility can not display.

On Windows, you can use PuTTY or RealTerm.

On a MAC, you can use [something else].

When you use a Terminal program, make sure that you know the communication port name and baud before opening the program. Also be sure that the port is not already open in another program. A port can only be open is one place at a time!

The Terminal programs vary in use, but for example, launch Putty.exe. Set up your communication by:

• Selecting the 'Serial' radio button

Select the 'Serial' branch in the tree on the bottom left.

• Enter the name of your COM port in 'Serial line to connect to'
• Enter 115200 in 'Speed (baud)'
• Change 'Flow control' from 'XON/XOFF' to 'None'

Go back to the 'Session' screen by selecting the 'Session' in the top left of the tree.

• Name your settings in 'Saved Sessions' and click 'Save'
• Click the 'Open' button

Processing

Processing is a free IDE that is easy to use, makes nice graphics, and easily performs serial communication. The NU32_Utility was written in Processing. See the Processing page for more specific information on Processing.

The following code is a simple Processing sketch that will open a serial port and print any characters received to the Debug area.

    Processing code

To send characters to the NU32 with Processing, try the following:

more code

Processing has the ability to check for incoming characters in parallel to running code in the Draw() routine. Try the following:

more code

MATLAB

MATLAB can open a serial port and send and receive data as well. This is useful when you need to plot data, perform a complicated mathematical function, or error check a function that you wrote in C to the function in MATLAB.

To open a port in MATLAB:

open

To send data:

code

To receive data:

code

Put together, the code looks like:

code

More Information

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