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Network and Systems Laboratory
nslab.ee.ntu.edu.tw
Network and Systems Laboratory
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GPIO on MSP430
 6 ports on MS430F1611
 Port 1
 Port 2
 Port 3
 Port 4
 Port 5
 Port 6
 Each port has 8 pins
 Px.0 ~ Px.7
Network and Systems Laboratory
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Multiplexed
 Port pins are often
multiplexed
 Means it may have more
than one function
 Example: P1.0/TACLK
 It can be P1.0 GPIO
 Or it can be TACLK
 You must select the
function you want
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Select Direction
 Each pin can be configured as input or output
 When set to output
 You can configure it to be
High: 3.3V
 High: voltage is Vcc (Supply voltage)
Low: 0V
 Low: voltage is GND (Ground)
Network and Systems Laboratory
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Input
 When you select a pin as input direction, a
corresponding bit in peripheral register will
 Set to 0 when input voltage is low
 Set to 1 when input voltage is high
High: 3.3V
1
Low: 0V
0
Network and Systems Laboratory
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Interrupts
 Only P1 and P2 are interruptible
 For each pin in P1 and P2, you can enable or
disable its interrupt
 Enable means it will detects interrupt
 Disable means nothing happen when interrupt
occur
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How to Detect Interrupts
 For GPIO, interrupt is detected when a transition
occur
 Low to high transition:
 High to low transition:
3.3 V
0V
3.3 V
0V
 You must define which one you want to detect
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Interrupt Flag
 When the MCU detects an interrupt
 A corresponding bit in peripheral register will set to
1
 Branch to ISR
 For some interrupt, you must clear the GPIO
interrupt flag in software
 Means you must set the bit to 0
 or the program will re-enter the ISR again
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GPIO Registers
 Each GPIO port has four registers
 Input: PxIN
 Output: PxOUT
 Direction: PxDIR
 Port Select(function select): PxSEL
 P1 and P2 have three more
 Interrupt flag: PxIFG
 Interrupt edge select: PxIES
 Interrupt enable: PxIE
 Each register is 8-bit long
 x represent the port number
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How to Select Function
 You want to select this pin as
GPIO(P1.0) function
 The register related to function select
 Port Select(function select): PxSEL
 This is port 1, so the related register
is P1SEL
 From user guide
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How to Select Function
 P1SEL is 8-bit long
 Each bit corresponding to a pin in the
port
7
P1SEL
x
0
x
P1.6/TA1
P1.7/TA2
x
x
x
P1.4/SMCLK
P1.5/TA0
x
x
P1.2/TA1
P1.3/TA2
x
P1.0/TACLK
P1.1/TA0
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How to Select Function
7
P1SEL
x
0
x
P1.6/TA1
P1.7/TA2
x
x
x
P1.4/SMCLK
P1.5/TA0
x
x
P1.2/TA1
P1.3/TA2
x
P1.0/TACLK
P1.1/TA0
 You want select P1.0/TACLK as P1.0
 Set the corresponding bit to 0
7
P1SEL
x
0
x
x
x
x
x
 In C: P1SEL = P1SEL & 0xFE;
x
0
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Setting Bits
Hexadecimal
 P1SEL = P1SEL & 0xFE;
 P1SEL &= 0xFE;
 Usually, we use hexadecimal in setting registers
 0xFE = 11111110 (binary number)
 Why P1SEL &= 0xFE;
 Why not P1SEL = 0xFE;
 The other registers are similar
 check user guide
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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 ISR
 How do you know which one generate the
interrupt
 check interrupt flag
 Ex. if(PxIFG & 0x01)
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Hardware
 LEDs
 Switches
 Keypad
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LEDs
 This is a typical connection
 When P1.1 set to high
 DVcc = VP1.1
 no current flow
DVcc
Vanode Vcathode
 When P1.1 set to low
 VP1.1 = 0
 Current flow through, turn on the LED
 GPIO can use as an On/Off control
R
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Control LEDs
 Look at Taroko schematic
 which pins the LEDs connected to
 These pins should be input direction or output
direction?
 How to set these pins to high (or low)
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Will This Work
R
Vcathode
Vanode
 Maybe, but you shouldn’t do this
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Max Current Source/Sink
 Each 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 should not use those pin as a power source (or
sink) for any sensor/circuit directly
Network and Systems Laboratory
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Transistors
 One of the fundamental building block of ICs
 Two type: NPN, PNP
 B: Base
 C: Collector
 E: Emitter
 We will talk about NPN transistor
 most commonly used
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Functional Model
 A 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')
Network and Systems Laboratory
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Transistor as An On/Off Switch
 Application Circuit 
 Load is turn on when chip
output is high
 Choose a proper transistor and
base resistor RB
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How To Choose
 Transistor
 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
Network and Systems Laboratory
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Switches
A
B
 Operation
 Open: A and B are not connected in normal state
 Close: When you press the button, A and B are
connected
 Typical circuit
 When the switch is open, voltage of
USERINT stay at high
 When the switch is close, voltage of
will be low
Network and Systems Laboratory
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Pull-up Resistor
 R4 is a pull-up resistor
 keep USERINT at high
 What if we don’t have R4
 When it is open, USERINT is still high
 When it is close

Short circuit
USERINT
Network and Systems Laboratory
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Typical Value
 Typical value of pull-up resistor
 10K ohm, 47K ohm, 100K ohm, 1M ohm
 Amount of current flow through when it close
 I = V/R; Vcc = 3.3V, R = 10K ohm
 I = 0.00033A = 330 μA
 330 μA, does it matter
 for some type of switch, it is ok

close time very short
 but not all, eg. reed switch
Network and Systems Laboratory
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Value of Pull-up
 To save power, can we use very large pull-up
resistor?
 ans: No
Network and Systems Laboratory
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Impedance
 Resistance: R = V/I
 relation ship between the magnitude of the voltage
and current
 Impedance: Z = V/I (these are complex number)
 relation ship between the magnitude and phase of
the voltage and current
Network and Systems Laboratory
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Input Impedance
This is a simplified analysis, we
consider impedance as a
resistance. Impedance is more
complex than a simple
resistance
Vcc
R1
Z Ri
Ri
Network and Systems Laboratory
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Resistors in series
 Ri is the input impedance of MSP430 GPIO
pin
 TI didn’t specify its value
 but it should be larger than 10M ohm
 R1 is pull-up resistor
 if R1 is too large – R1 = Ri
 then VMCUpin = Vcc / 2
 it is no longer a high state
Vcc
R1
VMCUpin
Ri
Network and Systems Laboratory
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Debounce Capacitor
 C9 is a debounce capacitor
 Bounces
 It could generate more than one interrupts when
the switch is pressed
Network and Systems Laboratory
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HW/SW debounce
 Hardware debounce
 Capacitor will charge when the
switch is open, and it will discharge
when the switch is close (pressed)
 Pros: smooth the line
 Cons: increase the response time
 Software debounce
 Delay some time in the ISR
 How long

you have to try out
Network and Systems Laboratory
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Keypad
 This is a 3x4 matrix
keypad
 When you press a
button, a pin in X
and a pin in Y is
connected
 How to interface to
MSP430?
Network and Systems Laboratory
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Today’s Labs
1. Flash a LED on Taroko
2. Toggle a LED when UserInt switch on Taroko
pressed
3. Control on/off of a LED by UserInt switch, control
another by reed switch
4. Connected a 3x4 Keypad to Taroko, flash the LEDs
Red Green Yellow
Flash once
Flash twice
Network and Systems Laboratory
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JTAG Driver
 C:\Program Files\IAR Systems\Embedded
Workbench Evaluation
4.0\430\drivers\TIUSBFET\WinXP
Network and Systems Laboratory
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IAR Setting
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IAR Setting
Network and Systems Laboratory
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Network and Systems Laboratory
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Network and Systems Laboratory
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Network and Systems Laboratory
nslab.ee.ntu.edu.tw
Network and Systems Laboratory
nslab.ee.ntu.edu.tw
Network and Systems Laboratory
nslab.ee.ntu.edu.tw
Network and Systems Laboratory
nslab.ee.ntu.edu.tw
Network and Systems Laboratory
nslab.ee.ntu.edu.tw