Simple Digital I/O in C

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Simple Digital I/O in C
These lecture notes created by Dr. Alex Dean, NCSU
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In these notes . . .

• Simple Digital I/O
• Port
• Data Direction
• Data
• Drive Capacity
• Use
• Initialization
• Reading
• Writing
• Example
• Echo LEDs

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Digital Input/Output (I/O) Ports

• The fundamental interfacing subsystem
• A port bit can be input or output
• M30626 has 11 Programmable I/O Ports (total of 87
  digital I/O bits) (P1 through P7 and P9 and P10
  are bidirectional 8-bit ports, while P8 has an an
  input-only bit)
• For some other MCUs some ports may be limited to
  only input or output
• Direction register sets bit direction (PD1, etc.)
• 1 Output
• 0 Input (value of direction register bits after
  reset)
• Data register holds actual data
• P1 etc.
• M16C62P Hardware Manual, Programmable I/O
  Ports

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Programmable I/O Port
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Digital I/O Port as Input
enabled, turns on pull-up when input is 1
0
1
Read direction
1
off
0
0
Read port
disabled
1
off
Read port
enabled
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Digital I/O Port as Output
disabled
1
0
Read direction
1
1
Read port
enabled
0
1
enabled, behave as inverters
Read port
disabled
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Pull-Up Resistors for Inputs

• Used to simplify interfacing with devices with
  limited signal swing
• M30626 is digital CMOS and is only designed to
  operate correctly with valid input voltages
  (Everything else is illegal and is not
  guaranteed)
• Logic 1 0.8 VCC to VCC, Logic 0 0V to 0.2
  VCC
• Resistor is used to pull up voltage of signal
  from devicewith two states
• Low resistance to ground
• High resistance (essentially open circuit)
• Pull-up resistor is built into microcontroller to
  simplify circuit design and eliminate external
  components (save money, space, assembly effort)
• M30626 Pull-Ups
• Controlled in blocks of 4 (upper and lower halves
  of each port)
• Each block is enabled by a bit in PUR0, PUR1 or
  PUR2
• Pull-ups disabled if a port bit is configured as
  an output
• Value typically 120kW, min 66 kW, max 500kW
• M16C62P Hardware Manual, Programmable I/O Ports.

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Example in Assembly P6 Echoes Nibble Data

• Configuring port to desired structure
• Top 4 bits (4-7) of P6 are inputs
• Clear bits 4-7 of PD6
• These inputs need pull-up resistors
• Set bit PU15 of special function register (SFR)
  PUR1 to enable pull-ups
• Bottom 4 bits of P6 are outputs
• Set bits 0-3 of PD6

Bits 7-4
Port 6
Bits 3-0
Init or.b PU15, PUR1 mov.b 00001111b,
PD6 Loop mov.b P6, R0 read inputs shl.b -4,
R0 move bits 7-4 into 3-0 mov.b R0, P6 write
outputs jmp Loop repeat forever
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C-Level Support for SFR Interfacing

• Renesas has provided C support for MCU special
  function registers and special bits in sfr62p.h
• Original is in Renesas\QSK62P\Sample_Code\Common
• File is copied into project directory when you
  create a new file
• Note that these names are lower-case!
• Can treat SFRs (and some bits) as variables
• Examples
• To initialize a GPIO ports direction, set the
  port data direction register
• pd0 is the name for SFR PD0
• Write all 0s (0x00) to it to make it an input
  port
• pd0 0x00
• Write all 1s (0xff) to make it an output port
• pd1 0xff
• To read from the port
• data p0
• To write to a port
• p1 data
• Can even access some bits (use names in sfr62p.h)
• p1_1 0
• n p0_7

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Example in C P6 Echoes Nibble Data
include sfr62p.h define DIR_OUT (1) define
DIR_IN (0) unsigned char a pu15 1 / pd6
0xf0 / pd6_0 pd6_1 DIR_OUT pd6_2 pd6_3
DIR_OUT pd6_4 pd6_5 DIR_IN pd6_6 pd6_7
DIR_IN while (1) a p6 a gtgt 4 p6 a

• Reading and writing data
• Load data from input port
• Move top nibble to bottom
• Write data to output port
• Jump back to start
• Now lets invert the data before writing it out
• Load data from input port
• Move top nibble to bottom
• Invert it (complement)
• Write data to output port
• Jump back to start

Bits 7-4
a a
Port 6
Bits 3-0
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Example Application Revisiting the Response Timer

• Requirements
• Measure time delay between when program lights
  LED and user presses switch
• Display delay on LCD
• Input
• Switch active low, needs pull-up
• Outputs
• LED active low, triggers user
• LCD displays time measurement
• Development plan
• Get a simple program skeleton to compile follow
  tutorial 1 or 2 if needed
• Verify debugger takes us to main()
• Configure and test LED outputs
• Configure and test switch inputs
• Configure and test LCD
• Create and test time delay measurement function