Transcript Digital I/O: Connecting the Outside World

Digital I/O
Connecting to the Outside World
Programming Unit, Lecture 4
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Digital I/O
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Digital I/O is an essential requirement for most
micro-controller applications.
•Digital I/O: input/output of digital “on-off” bit or group of bits from
the CPU to outside world.
•User interface controls. Configuration option switches. Activate
indicators such as lamps or LEDs, sound alerts or alarms, control
motors, solenoids, or valves.
•Read or monitor the state of external switches or sensors.
•Digital communication with other external IC’s or devices.
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•BASIC Stamp has 16 I/O pins.
•Labeled P0 thru P15.
Referenced as pin n in software,
where n can be 0 to 15.
•Bi-directional, can be defined in
software as inputs or outputs
with the INPUT and OUTPUT
instructions.
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1
•For example,
INPUT pin5
defines logical pin5
(or P5) as an output
pin. Note that P5
corresponds to
physical pin 10 on
the actual BASIC
Stamp IC. Be sure
not confuse logical
pins with physical
pins.
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Digital I/O pins can be used for connection to external
circuits or devices but care must be exercised to avoid
damaging or destroying the BASIC Stamp IC.
Observe Electrostatic Discharge precautions, particularly
when the BASIC Stamp is not installed on a circuit board
or the I/O pins are unconnected (“floating”).
Never connect the pins to voltages outside the range of 0
to 5 volts. Observe polarities!
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Pull Up and Pull Down Resistors
•BASIC Stamp I/O pins defined as inputs have high-impedance. Pins
can be in an unpredictable HIGH or LOW state when charged are
discharged by leakage paths on the printed circuit board.
•Many misbehaving CMOS circuits or micro-controller programs can
be traced to unconnected input pins.
•All inputs should be properly terminated with a high value resistor to
either VCC (a pull up) or GND (pull down). Termination to VCC
with a pull up resistor is usually the preferred method.
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Pull Up and Pull Down Resistors
The resistance value for pullup resistors is not usually critical. 10k
ohms is common but can be from 1k to 100k ohms.
With VCC=5V and a pullup resistor of 10k, the maximum current
is 500 microamperes or 0.5 ma.
BalloonSat has 10k pullup resistors installed (10-pin SIP).
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Digital Output
•Set the I/O direction for output with the OUTPUT pin instruction.
•A pin configured as an output can be set to a logical high or low state
with the HIGH pin or LOW pin instructions.
•A logical LOW is nominally 0 volts.
–This state is also called L, 0, off, reset or clear
•A logical HIGH is nominally 5 volts.
–This state is also called H, 1, on, set or asserted.
•I/O pins are limited in their current sinking and sourcing ability.
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I/O Pin as a Current Source
• When pin is HIGH, current flows from the
pin through the load to GND.
•
•When pin is LOW, no potential difference
between pin and GND, no current flows.
The current limiting
resistor may be needed
in some applications.
•The instruction HIGH pin would activate
the load.
•The instruction LOW pin turns the load off.
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I/O Pin as a Current Sink
• When pin is LOW, current flows from the
+5V power supply through the load to I/O pin.
• When pin is HIGH, no potential difference
between pin and +5V, no current flows.
The current limiting
resistor may be
needed
in some applications.
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•The instruction LOW pin would activate the
load.
•The instruction HIGH pin turns the load off.
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Digital Output Applications
Driving an LED with
I/O pin as a current
source.
Driving an LED with
I/O pin as a current
sink.
HIGH pin turns LED on.
LOW pin turns LED on.
Resistor R limit is chosen to limit current to a safe and appropriate
value for the BASIC Stamp IC and the LED.
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Digital Output Applications
The BASIC Stamp I/O pins have limited current sink and
source capability. The pin’s safe voltage limits are 0 to +5V.
I/O pins can safely sink or source about 20 mA of current.
For applications requiring higher voltage or current, external
buffering is required. The BASIC Stamp can be used to drive
bipolar transistors, FETs or solid state or electromechanical
relays which in turn can control high current or high voltage
circuits.
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Buffered
Digital
I/O Examples
Digital
Output
Applications
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Digital Input
•Set the I/O direction for output with the INPUT pin instruction.
•A variable assigned to the I/O pin will take on a value reflecting
the voltage level at the pin.
This will work but there is a
better way!
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Digital I/O: reading a switch
Common Switch Types:
•Normally Open (NO)
•Normally Closed (NC)
•Toggle
Mechanical switches are not perfect and may
•Momentary (Pushbutton)
exhibit contact bounce as the switch is operated.
•Rotary
Typically on the time scale of milliseconds.
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Input Interfacing of Electronics to BASIC Stamp
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Analog I/O with the BASIC Stamp
•The BASIC Stamp’s I/O pins are digital and can only be on or off.
The Stamp has no direct ability to read or generate analog voltage
levels.
•Additional hardware can be added at a price: $’s and complexity.
–Analog-to-Digital Converters (ADC)
–Digital-to-Analog Converters (DAC)
•For simple analog input or output applications, the digital I/O pins
and the technique of pulse-width modulation (PWM) can “fake”
analog I/O.
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Analog Output using PWM
•A simple Digital-to-Analog Converter can be built with an RC network
and a single instruction on the BASIC Stamp.
•The I/O pin can only be High (+5V) or
Low (0 volts) but the RC network will
“average” a train pulses to generate an
analog voltage.
•The PWM instruction is a built-in
command to send pulses to the RC
network.
•Format of the command is
PWM pin, Duty, Duration
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•Pin selects the desired I/O pin.
•Duty (duty cycle) specifies the ratio of the high pulse duration to the
duration of all pulses in a specified interval of time.
•Duration specifies the duration of the PWM pulse train.
Duty is on a scale of 0 to 255, corresponding to 0 to 5 volts. If we
wanted an analog voltage output of 2.5 volts,
duty = 255 x 2.5 / 5
duty = 127.5 which we round to the integer 128.
The required Duration is about 5 time-constants of the RC network,
Duration = 5 x 10K x 1 uF
Duration = 0.05 sec or 50 milliseconds
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•The PWM technique is inexpensive and quick to implement but it
suffers from limited resolution and cannot supply any significant current
to a load.
•An external buffer using an operational amplifier (Opamp) can be used
to improve performance.
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Analog Input using PWM
•The BASIC Stamp has no capability for directly reading analog
quantities but it does have a specialized instruction RCTIME which
allows determining a resistance or capacitance value by measuring the
time it takes to charge or discharge an RC network.
•The I/O pin is set to +5v, charging
capacitor C. The pin is then changed to
an input and a counter measures the
time to discharge C through R.
•The 220 ohm resistor limits current
from the I/O pin to a safe value.
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Circuit for measuring
discharge time.
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Analog Input using PWM
•The syntax for using RCTIME is
RCTIME pin, state, variable
pin is the selected I/O pin
state specifies the state of pin for the measurement
0 sets pin to 0v or logical Low initially (measure charge time)
1 set pin to 5v or logical High initially (measure discharge time)
variable is the name of a variable into which the counter is stored
See the PBASIC Help file for additional details.
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