<< Chapter < Page Chapter >> Page >

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

Questions & Answers

can someone help me with some logarithmic and exponential equations.
Jeffrey Reply
sure. what is your question?
ninjadapaul
20/(×-6^2)
Salomon
okay, so you have 6 raised to the power of 2. what is that part of your answer
ninjadapaul
I don't understand what the A with approx sign and the boxed x mean
ninjadapaul
it think it's written 20/(X-6)^2 so it's 20 divided by X-6 squared
Salomon
I'm not sure why it wrote it the other way
Salomon
I got X =-6
Salomon
ok. so take the square root of both sides, now you have plus or minus the square root of 20= x-6
ninjadapaul
oops. ignore that.
ninjadapaul
so you not have an equal sign anywhere in the original equation?
ninjadapaul
Commplementary angles
Idrissa Reply
hello
Sherica
im all ears I need to learn
Sherica
right! what he said ⤴⤴⤴
Tamia
what is a good calculator for all algebra; would a Casio fx 260 work with all algebra equations? please name the cheapest, thanks.
Kevin Reply
a perfect square v²+2v+_
Dearan Reply
kkk nice
Abdirahman Reply
algebra 2 Inequalities:If equation 2 = 0 it is an open set?
Kim Reply
or infinite solutions?
Kim
The answer is neither. The function, 2 = 0 cannot exist. Hence, the function is undefined.
Al
y=10×
Embra Reply
if |A| not equal to 0 and order of A is n prove that adj (adj A = |A|
Nancy Reply
rolling four fair dice and getting an even number an all four dice
ramon Reply
Kristine 2*2*2=8
Bridget Reply
Differences Between Laspeyres and Paasche Indices
Emedobi Reply
No. 7x -4y is simplified from 4x + (3y + 3x) -7y
Mary Reply
is it 3×y ?
Joan Reply
J, combine like terms 7x-4y
Bridget Reply
im not good at math so would this help me
Rachael Reply
yes
Asali
I'm not good at math so would you help me
Samantha
what is the problem that i will help you to self with?
Asali
how do you translate this in Algebraic Expressions
linda Reply
Need to simplify the expresin. 3/7 (x+y)-1/7 (x-1)=
Crystal Reply
. After 3 months on a diet, Lisa had lost 12% of her original weight. She lost 21 pounds. What was Lisa's original weight?
Chris Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
China
Cied
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
many many of nanotubes
Porter
what is the k.e before it land
Yasmin
what is the function of carbon nanotubes?
Cesar
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
AMJAD
preparation of nanomaterial
Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
AMJAD
what is system testing
AMJAD
what is the application of nanotechnology?
Stotaw
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
Azam
anybody can imagine what will be happen after 100 years from now in nano tech world
Prasenjit
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
Azam
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
Prasenjit
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
Damian
silver nanoparticles could handle the job?
Damian
not now but maybe in future only AgNP maybe any other nanomaterials
Azam
can nanotechnology change the direction of the face of the world
Prasenjit Reply
At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
Ali Reply
the Beer law works very well for dilute solutions but fails for very high concentrations. why?
bamidele Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
Privacy Information Security Software Version 1.1a
Good
Got questions? Join the online conversation and get instant answers!
QuizOver.com Reply

Get the best Algebra and trigonometry course in your pocket!





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
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Intro to computational engineering: elec 220 labs' conversation and receive update notifications?

Ask