Lab 1: GPIO

Network and Systems Laboratorynslab.ee.ntu.edu.tw


GPIO on MSP4306 ports on

MS430F1611Port 1Port 2Port 3Port 4Port 5Port 6

Each port has 8 pinsPx.0 ~ Px.7


MultiplexedPort pins are often

multiplexedMeans it may have

more than one functionExample: P1.0/TACLK

It can be P1.0 GPIOOr it can be TACLK

You must select the function you want


Each pin can be configured as input or output

When set to outputYou can configure it to beHigh: voltage is Vcc (Supply voltage)Low: voltage is GND (Ground)

High: 3.3V

Select Direction

Low: 0V


InputWhen you select a pin as input direction, a

corresponding bit in peripheral register willSet to 0 when input voltage is lowSet to 1 when input voltage is high

High: 3.3V

Low: 0V

1

0


InterruptsOnly P1 and P2 are interruptibleFor each pin in P1 and P2, you can enable or

disable its interruptEnable means it will detects interruptDisable means nothing happen when interrupt

occur


How to Detect InterruptsFor GPIO, interrupt is detected when a

transition occurLow to high transition:

High to low transition:

You must define which one you want to detect

0 V

3.3 V

0 V

3.3 V


Interrupt FlagWhen the MCU detects an interrupt

A corresponding bit in peripheral register will set to 1

Branch to ISRYou must clear the GPIO interrupt flag in

softwareMeans you must set the bit to 0or the program will re-enter the ISR again


GPIO RegistersEach GPIO port has four registers

Input: PxINOutput: PxOUTDirection: PxDIRPort Select(function select): PxSEL

P1 and P2 have three moreInterrupt flag: PxIFGInterrupt edge select: PxIESInterrupt enable: PxIE

Each register is 8-bit longx represent the port number


How to Select FunctionYou want to select this pin as

GPIO(P1.0) functionThe register related to function

selectPort Select(function select): PxSELThis is port 1, so the related

register is P1SELFrom user guide


How to Select FunctionP1SEL is 8-bit long

Each bit corresponding to a pin in the port

P1SEL x x x x x x x x

07

P1.0/TACLK

P1.1/TA0

P1.2/TA1

P1.3/TA2

P1.4/SMCLK

P1.5/TA0

P1.6/TA1

P1.7/TA2


How to Select Function

You want select P1.0/TACLK as P1.0Set the corresponding bit to 0

In C: P1SEL = P1SEL & 0xFE;

P1SEL x x x x x x x x

07

P1.0/TACLK

P1.1/TA0

P1.2/TA1

P1.3/TA2

P1.4/SMCLK

P1.5/TA0

P1.6/TA1

P1.7/TA2

P1SEL x x x x x x x 0

07


Setting BitsP1SEL = P1SEL & 0xFE;

P1SEL &= 0xFE;Usually, we use hexadecimal in setting

registers0xFE = 11111110 (binary number)

Why P1SEL &= 0xFE; Why not P1SEL = 0xFE;

The other registers are similarcheck user guide

Hexadecimal


GPIO Interrupt All P1 pins source a single interrupt vector,

and all P2 pins source a different single interrupt vector.Interrupts generated by P1.0, P1.1, …, P1.7 all

go to same ISRHow do you know which one generate the

interruptcheck interrupt flag


Hardware LEDsSwitchesKeypad


LEDsThis is a typical connectionWhen P1.1 set to high

DVcc = VP1.1

no current flowWhen P1.1 set to low

VP1.1 = 0Current flow through, turn on the LED

GPIO can use as an On/Off control

DVcc

Vanode Vcathode

R


Control LEDsLook at Taroko schematic

which pins the LEDs connected toThese pins should be input direction or output

direction?How to set these pins to high (or low)


Will This Work

Maybe, but you shouldn’t do this

VanodeVcathode

R


Max Current Source/SinkEach pin has a maximum amount of current it

can provide(source) or accept(sink)For MSP430

Max source: 6mA Max sink: 6mA

Usually it can sink more current than it can source

You cannot use those pin as a power source (or sink) for any sensor/circuit directly


TransistorsOne of the fundamental building block of ICsTwo type: NPN, PNP

B: BaseC: CollectorE: Emitter

We will talk about NPN transistormost commonly used


Functional ModelA base current IB flows only when the

voltage VBE across the base-emitter junction is 0.7V or more

Ic = hFE × IB (hFE is current gain)

The collector-emitter resistance RCE is controlled by the base current IB: IB = 0 RCE = infinity transistor off

IB small RCE reduced transistor partly on

IB increased RCE = 0 transistor full on ('saturated')


Transistor as An On/Off SwitchApplication Circuit Load is turn on when chip

output is highChoose a proper transistor

and base resistor RB


How To ChooseTransistor

Two parameter: Ic and hFE

Ic(max) must be larger than Iload (Ic)

Ic = hFE × IB hFE(min) > 5 x ((Ic) /(maximum output

current from the chip))RB

RB = (Vcc × hFE) / (5 × Ic)choose the nearest standard value

resistor


SwitchesOperation

Open: A and B are not connected in normal state

Close: When you press the button, A and B are connected

Typical circuitWhen the switch is open, voltage of USERINT stay at highWhen the switch is close, voltage of will be low

A B


Pull-up ResistorR4 is a pull-up resistor

keep USERINT at high

What if we don’t have R4When it is open, USERINT is still highWhen it is close

Short circuitUSERINT


Typical ValueTypical value of pull-up resistor

10K ohm, 47K ohm, 100K ohmAmount of current flow through when it close

I = V/R; Vcc = 3.3V, R = 10K ohmI = 0.00033A = 330 μA

330 μA, does it matterfor some type of switch, it is ok

close time very shortbut not all, eg. reed switch


Value of Pull-upTo save power, can we use very large pull-up

resistor?ans: No


ImpedanceResistance: R = V/I

relation ship between the magnitude of the voltage and current

Impedance: Z = V/I (there are complex number)relation ship between the magnitude and phase

of the voltage and current


Input Impedance

Z Ri

R1

Ri

Vcc

This is a simplified analysis, we consider impedance as a

resistance. Impedance is more complex than a simple

resistance


Resistors in seriesRi is the input impedance of MSP430

GPIO pinTI didn’t specify its valuebut it should be larger than 10M ohm

R1 is pull-up resistorif R1 is too large – R1 = Ri

then VMCUpin = Vcc / 2it is no longer a high state

R1

Ri

Vcc

VMCUpin


Debounce CapacitorC9 is a debounce capacitorBounces

It could generate more than one interrupts when the switch is pressed


HW/SW debounceHardware debounce

Capacitor will charge when the switch is open, and it will discharge when the switch is close (pressed)

Pros: smooth the lineCons: increase the response time

Software debounceDelay some time in the ISRHow long

you have to try out


KeypadThis is a 3x4

matrix keypadWhen you press a

button, a pin in X and a pin in Y is connected

How to interface to MSP430?


Today’s Labs1. Flash a LED on Taroko2. Toggle a LED when UserInt switch on Taroko

pressed3. Control on/off of a LED by UserInt switch, control

another by reed switch4. Connected a 3x4 Keypad to Taroko, flash the

LEDsRed GreenYellow

Flash once

Flash twice


JTAG DriverC:\Program Files\IAR Systems\Embedded

Workbench Evaluation 4.0\430\drivers\TIUSBFET\WinXP


IAR Setting


IAR Setting









Documents

Lab 1: GPIO