Parallel ports, power supply, and
the clock oscillator
Chapter Three
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Outline
• Parallel ports
– Technical challenges
– Connecting to the parallel port
– The PIC 16F84A parallel ports
• Power supply
• Clock oscillator
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Data Transfer
• Almost any embedded system needs to transfer
digital data between its CPU and the outside world.
– Direct user interface, including switches, keypads, lightemitting diodes (LEDs) and displays
– Input measurement information, from external sensors,
possibly being acquired through an analog-to-digital
converter
– Output control information, for example to motors or
other actuators
– Bulk data transfer to or from other systems or subsystems,
moving in serial or parallel form, for example sending
serial data to an external memory.
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Output Parallel Ports
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Input Parallel Ports
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Bi-directional Parallel Ports
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Port electrical characteristics
Modeling a logic gate output.
(a) Generalized model. (b) Model of CMOS logic gate output
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Schmitt trigger inputs
Schmitt trigger characteristics.
(a) Buffer with Schmitt trigger input. (b) Input/output characteristic
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The ‘Open Drain’ output
(a) An ‘Open
Drain’ output.
(b) Open Drain
output driving
load resistor.
(c) The ‘WiredOR’ connection
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Connecting to the parallel port
(1) Switches
(a) SPDT connection. (b) SPST with pull-up resistor.
(c) SPST with pull-down resistor
Pull-up values in the range 10–100 kΩ
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Connecting to the parallel port
(2) Light-emitting diodes
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Connecting to the parallel port
(2) Light-emitting diodes
Driving LEDs from logic gates.
(a) Gate output sourcing current to LED
(b) Gate output sinking current from LED
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Connecting to the parallel port
(2) Light-emitting diodes
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The PIC 16F84A parallel ports
• Port A – 5 Bits
– RA3:RA0
– RA4/T0CKI
• Port B – 8 Bits
– RB0/INT
– RB3:RB1
– RB7:RB4: Interrupt on change
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Port output characteristics -1
R = 130 Ω
VOH vs. IOH (VDD = 3V, −40 to 125◦C)
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Port output characteristics -2
VOL vs. IOL (VDD = 3V, −40 to 125◦C)
R = 36 Ω
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The clock oscillator
• Faster clock gives faster execution, but more
power consumption.
• The clock oscillator must give stable and
accurate clock signal.
• Oscillator types:
– Resistor–capacitor (RC)
• Not precise
– Crystal or ceramic
• Precise frequency, fragile, should be near the MC
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Oscillator types
(a) Resistor–capacitor (RC).
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(b) Crystal or ceramic
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The 16F84A clock oscillator
• Types:
1) XT – crystal: 1-4 MHz
2) HS – high speed: >= 4 MHz, with ceramic
resonators.
3) LP – low power: <= 200 KHz, e.g., 32.768 kHz
(i.e. 215),
4) RC – resistor-capacitor
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(a) Crystal or ceramic, HS, XT or LP. (b) Resistor–
capacitor. (c) Externally supplied clock
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Data Sheet Information
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Power Supply
RC
Oscillator
100 nF
decoupling
capacitor
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16F84A operating conditions
Summary – 1
• The parallel port allows ready exchange of digital data
between the outside world and the controller CPU.
• It is important to understand the electrical
characteristics of the parallel port and how they
interact with external elements.
• While there is considerable diversity in the logic design
of ports, they tend to follow similar patterns.
• The internal circuitry is worth understanding, as it
leads to effective use of ports.
• The 16F84A has diverse and flexible parallel ports.
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Summary – 2
• A microcontroller needs a clock signal in order to
operate. The characteristics of the clock oscillator
determine speed of operation and timing stability, and
strongly influence power consumption. Active
elements of the oscillator are usually built in to a
microcontroller, but the designer must select the
oscillator type, and its frequency and configuration.
• A microcontroller needs a power supply in order to
operate. The requirements need to be understood and
must be met by a supply of the appropriate type.
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