ICT2104 Chapter 3: GPIO

ICT2104 Chapter 3: GPIO

1. Basic I/O Interfacing Concepts

   • I/O Ports: Physical electrical links for microcontroller to communicate with external world
   • Types: Digital & Analogue
   • Mostly multi-function: Normal I/O pin or special function pin
   • Each digital I/O port...
     • Can have Schmitt Trigger
       • An active circuit which converts analog to digital signal
       • 'Trigger' because output retains value until input changes sufficiently to 'trigger' a change
     • Combines input, output and interrupt functionality
     • Can be programmed for each bit (serial) or as an 8-bit port (parallel)

2. External Hardware Consideration

   • Output Port
     • Active Low / Normally High
       • 0 to turn on LED
       • 0 when switch is pressed
       • MCU - Resistor - Vcc
       • Current Sink Mode
     • Active High / Normally Low
       • 1 to turn on LED
       • 1 when switch is pressed
       • MCU - Resistor - Ground
       • Current Drive Mode
   • Input Port
     • External Pull-Up Resistor
     • Internal Pull-Up Resistor

3. MSP430 Digital I/Os

   • Up to 10 8-bit digital I/O ports (P1 to P10)
   • P1 and P2 have interrupt function
   • Can be multi-function pin

4. Port Direction, Schmitt, Pull Up/Down, Drive Strength, Function Select Registers

   • PxDIR: Direction of I/O
     • Bit 1: Output
     • Bit 0: Input
   • PxIN: For read only, reflects input signal
     • Bit 1: Input is high
     • Bit 0: Input is low
     • ! Avoid writing to them: results in increased current consumption
   • Schmitt Trigger
     • For slow edges to be converted to fast edges
     • Cleans up noisy signals
     • Changes waves to square waves
     • Reason: CMOS, BiCMOS and TTL devices require fairly fast edges on inputs
   • PxOUT: Value to write to output pin
     • Bit 1: Output is high
     • Bit 0: Output is low
   • PxREN: Enables/Disables Pull-Up/Down resistor of corresponding I/O Pin
     • Bit 1: Enabled
     • Bit 0: Disabled
     • If enabled...
       • PxOUT = 1: Pull-Up
         • Force it to be active low / normally high
         • Resistor near Vcc
       • PxOUT = 0: Pull-Down
         • Force it to be active high / normally low
         • Resistor near Gnd
     • Only for input pins (?)
   • PxDS: Select drive current strength for output ports
     • Bit 1: Full drive strength
     • Bit 0: Reduced drive strength
   • PxSEL: Select pin function for pins with other peripheral module functions
     • Bit 0: I/O function selected
     • Bit 1: Peripheral module function selected
   • Unused: Set to output

5. Multiplex and Polled I/O

   • Switch Problem: Debouncing - Voltage does not go instantly and cleanly between VSS and VCC when switch is pressed
     • Solution: Generate a delay period > 10ms before verifying keypress
     • Solution: SR latch-based switch debouncing circuit
     • Solution: RC network-based switch debouncing circuit
   • Multiplex Keypad: 4 Output Pins and 3 Input Pins
     • Getting key: Send voltage to output 1 by 1 and check if input is active 1 by 1
   • Polling: Constantly check status of port for new data
     • Simple to implement
     • Wastes CPU resources and energy

6. Handshaking and strobed I/O

   • Handshaking: Automated process of negotiation that dynamically sets parameters of communications channel established between 2 entities before normal communication begins
   • Handshake Control: To synchronise transfer of asynchronous data
     • Make use of Data Strobe signal to indicate that data has been sent
       • Can only be low after ACK* is received
     • ACK* to acknowledge data is received (device) and MCU can accept new data

7. Analogue I/O

   • System Components
     • Sensor: Converts analogue measurements into electrical signals (voltage/current)
     • Signal Conditioning: Filtering, Amplification, Attenuation or Impedance Transformation
   • Resolution: The smallest change to the analog voltage that can be converted to digital code
     • LSB: R = 1 / (2^n)
   • ADC: 12 bits, max data rate of 200 KSPS with 16 channels
     • tsample: time to sample and hold onto analogue input while it is being converted
     • tconvert: time it takes for conversion (13 x ADC12CLK)
     • Stores result in ADC12MEM register
     • Conversion limited by upper and lower limits