ELEC 332

In the Lab I: SIO to PC

Part 1: Connect RS232 Serial Interface

In addition to their role as ordinary digital port pins, P1.6 and P1.7 are also connected to the USI (Universal Serial Interface) module. This module supports I2C and SPI protocols for communication with peripheral chips, but not the asynchronous protocol used for serial I/O with a computer. However, we can implement this protocol in software, using these same two pins in their digital port role. These two pins are connected to J2, the connector on the right hand side of the CPU board.

The RS232 serial port of the PC uses voltages of +5 V and -5 V to represent 1 and 0. These need to be converted to the +3 V and 0 V levels used by the MSP432. Fortunately, there are a number of chips available which will do this with a minumum of fuss. We'll use the SP3223, as shown in the following circuit:


(Click to enlarge)

When assembled into a module for our ELEC332 breadboard, it looks like this:


(Click to enlarge)
If it looks like there's something important missing (i.e. the SP3223 chip), that's because it's on the bottom of the board. Since it's difficult to solder the pins of the 9-pin D connector to the top of the board, this reduces the number of vias required to layout the board. Also missing are four of the pins from the power connector. Since we only need +5 V and ground for this circuit, these were removed to make room for the DE-9 connector. Be sure to plug the module into the power socket correctly when installing it on the breadboard. Here's the other side of the module:

(Click to enlarge)


Connect the modules.

As mentioned above, the 5-pin connector on the right hand side of the CPU module plugs into the 5-pin connector on the left hand edge of this module. We will want to use the coax cable adapter to access the remaining I/O pins, so when everything is connected, it will look like this:


(Click to enlarge)

Note

Since the power connectors and data connectors are mutually perpendicular, you will have to unplug the CPU module from the breadboard, plug the serial module into it, and plug the resulting assembly back into the breadboard.


Connect to the PC serial port.

 Once you have the modules plugged together and firmly attached to the breadboard, connect a cable from the serial port of the PC to the 9-pin connector on the serial interface module.

Start and configure the Hyperterminal program.

 We eventually want to use this connection to allow programs on the PC to interact with programs on the MSP430. For now, we'll do this interaction manually, using the hyperterminal program on the PC (All Programs->Accessories->Communications->HyperTerminal). It will ask you a bunch of questions about location, which you can ignore.

Configure the terminal program.

 Select File->Properties from the menu and choose the "Connect To" tab. In the "Connect using:" field, choose COM1, then click the "Configure..." button. On the "Port Settings" tab, select
Bits per second:
9600
Data bits:
8
Parity:
None
Stop bits:
1
Flow control:
None
Click OK until all the dialogs go away. Hyperterminal is now poised for action.

Part 2: MSP430 to PC

Our first use of the serial interface will be to send data from the MSP430 to the PC. Since asynchronous SIO is not supported in hardware, we will have to generate the necessary signals in software. Fortunately, it is not too difficult. Even more fortunately, the required software has already been written and may be found, along with other software for this week's exercise in the lab3.zip file.


Build and load the program.

 Plug the USB debug interface into the CPU module, start up IAR Embedded Workbench, and open the lab3 workspace from this week's zip file. Build, download, and run the sio1 project.

Activate Hyperterminal.

 With the cursor over the Hyperterminal window, hit <Enter> on the PC keyboard. This will open the Hyperterminal connection. If all is well, you should see the alphabet repeatedly appear on the terminal screen.

The Hyperterminal program is a bit finicky about what it chooses to display and when it chooses to display it. Nothing will happen until it is connected, so be sure the box in the lower left corner of the window says "Connected" not "Disconnected." If it doesn't, try hitting <Enter> with the cursor in the window. The third box should say "9600 8-N-1". If not, bring up the configuration panel and make the necessary changes. Sometimes starting Hyperterminal and the MSP430 in a different order helps, as does hitting <Return> once both are running.

If in spite of your best efforts, nothing happens, check to be sure you are in fact generating an appropriate signal. Carefully probe the tx data line with the scope and verify that a signal with levels of about +5 V and -5 V is present. The signal on the tx data line is also put out on P1.2. Use the connector modules you used last week to connect this signal to the scope and verify that it looks as it should. The width of the pulses should be multiples of 1/9600 sec, or roughly 0.1 ms.

Programming Challenge.

 Even though it's happening really fast, typing the alphabet is not a very satisfying accomplishment for the amount of hardware we have assembled (and as we learned in 241, since the alphabet is a known quantity, sending it conveys no information). More impressive would be to send information that the MSP430 has gathered from the system in which it is embedded. So our first challenge is to build a digital voltmeter. Details are given in the Design Challenge section.

Part 3: PC to MSP430

Since we would like to be able to communicate in both directions between the MSP430 and the PC, we now have to get our connection working in the opposite direction, i.e. send data from the PC to the MSP430. This won't be quite so satisfying since we can't display characters on the MSP430. However, we can easilly display the bits of the characters on the digital output ports, so that's what we will do.


Build and load the program.

 Select the sio2 project from the lab3 workspace. This program reads a character and places its least significant bits on the P1 output port. Build, download, and run the program.

Send characters from PC to MSP430.

 With the cursor over the window of the Hyperterminal program, type a character on the keyboard. With the scope, probe the P1 output pins and verify that they have the correct values for the least significant 5 bits of the character you have typed. A quick test is to type sequential digits. The LED should come on for odd values and go off for even values.

Programming Challenge.

 Again, probing around with the scope to find out the bits of a transmitted character is not a very effective use of our fancy hardware. We want to be able to send data from the PC to the MSP430 and thence to a system we wish to control. In the coming weeks we'll be doing exactly that, controlling motors in response to commands from the PC. As hinted last week, we'll be using PWM to do it, so let's get some practice. Your challenge is to write a program which reads a number from the PC and produces a PWM waveform with the appropriate duty cycle. Again, details are in the Design Challenge section.