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So what?

In this lab we're going to use the MSP430's GPIO pins, combined with some external switches and an LED display, to build a basic I/O system for our board. Because of how things fit together on the board, it makes sense to use P1.0-P1.3 (the first three Port_1 GPIO Pins) to read the input switches and P1.4-P1.7 for the output signals.

Outputs

Setting up the outputs is easy-- simply set the upper four bits (bits 4-7) of &P1DIR to "1", and then write the output to the upper four bits of &P1OUT . That means you'll have to shift your data left 4 positions before output, but you should already know a simple technique to do so!
You'll notice that when you change the output, the corresponding input bits also change. This happens because the input hardware always reads the status of the line, regardless if it is set to input our output. Changing the &P1DIR values only connects or disconnects the driving circuitry built into the MSP430. In advanced applications this can be used to analyze potential faults in the circuitry outside the chip.

Inputs

Inputs are also "easy," but there are a few hardware concepts you'll need before you understand how they work!

A little bit about wires

As mentioned briefly in class, binary digital logic has two valid states, plus one third mystery state. That third state, "The High Impedance State," (High-Z) just means that the wire isn't connected to anything. You've already talked about using so called tri-state buffers to negotiate who can talk on a shared bus-- the listening components enter the high impedance state, allowing the transmitting component's signal to drive the bus with no conflicts.

Impedance is a generalized form of the classical Resistance concept. Impedances can be real or complex valued, and apply too signals expressed in complex exponential form ( whether constant or variable! ). To learn more about impedance, check out Dr. Johnson's sections from the Elec 241 course notes.

A basic switch

A simple SPST hardware switch

In order to read useful input from your switches, you need them to be "0" in one state, and "1" in the other. Yet knowing what you know about the third state, the switch shown above will actually give a "0"/"1" (depending on what you connect it to) when closed and "High-Z" when open. Because there's nothing else driving the sensor input besides our switch, the input value will be random when the switch is open . In digital logic this is called floating, and it is a very very bad thing.

One simple solution is the Pull-Up (or Pull-Down) Resistor . Connecting the floating side of the switch to a logic level through a large resistor will tie down the floating input when the switch is open, but won't effect the read value much when the switch is closed.

An spst hardware switch in the pulled up configuration.  One end of the switch is grounded, while the other is connected to the circuit input and Vcc through a large resistor.
As you can see, when the switch is closed, the input is shorted to ground and reads zero. When the switch is open, the pull-up resistor holds the previously floating end at Vcc.

Pull-ups in the msp430

For better or for worse, the MSP430 actually has pull up resistors already built into the chip's hardware. Configuring them takes several steps, but once setup they provide all the functionality above without the extra external connections.
  • Set the Pin Direction for P1.0-P1.3 to input. (Set bits 0-3 of &P1DIR to "0")
  • Enable the resistors themselves. (Set bits 0-3 of &P1REN to "1")
  • Configure the resistors to be pull-up. (Set bits 0-3 of &P1OUT to "1")
The most confusing part of the whole process is the double function of P1OUT . Because of the hardware implementation on the MSP430, &P1OUT controls the outputs as well as the connections to the pull up resistors. You will need to ensure that every time you output a value, you KEEP the lower four bits "1" . The easiest way to do this is just by ORing your raw output with the constant #0Fh before you write to P1OUT . The MSP430 does not have a specific "or" instruction by name, but bis does the same thing. For more info on bis and its inverse bic , see next week's lab.

The MSP430 resistor connection configuration.  Shows the double function of P1OUT
Notice that configured this way, the MSP430 GPIO pin takes the form of the simplified Pull-Up figure above.

Polling

    Philosophy

  • A traditional single threaded polling scheme consists of a main loop that runs continuously. Within that loop, the processor periodically checks for changes, and if there are none, continues looping. Once a change is detected, the program moves to a new section of code or calls a new subroutine to deal with the changes.
  • Polling has advantages and disadvantages-- it keeps program execution linear and is very easy to code and implement, but it also is not incredibly responsive. Since polling only checks values at certain points in the main run loop, if the loop is long or changes occur quickly, a polling scheme can miss input data. For now though it will suffice.

Assignment details

Your task is to code a simple input to output echo program for the MSP430. Your program should consist of:

  • A setup section that runs once and configures the GPIO pins
  • A main loop that runs infinitely
  • Code inside your loop to read the state of the GPIO input pins
  • A separate section of code to write the changes to the output pins and then return to the main loop

Masking

You should already know the basics of masking from class, but it becomes very important when dealing with I/O. Since different pins do different things in the same port (P1), you the programmer will have to be careful not to accidentally modify the wrong bits even though your instructions will operate on the entire register.

All images drawn by Matt Johnson, Rice ECE

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Source:  OpenStax, Intro to computational engineering: elec 220 labs. OpenStax CNX. Mar 11, 2013 Download for free at http://cnx.org/content/col11405/1.2
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