Pushbuttons on DE1-SoC Computer

Synchronization between HW/SW

• Synchronization between outside I/O world and the software running must be done carefully

  • Much to coordinate

• How does the processor know some in new input has arrived and also output has been sent?

  • Polling: processor explicitly asks the device if there is new data/sent data
  • Interrupt driven: processor diverts to different software upon interrupt signal

• Data register behaviour: when key i is pressed, bit i of the data register goes to 1

  • When released, bit i goes to 0
  • But the processor could miss it

• Edge capture register behaviour: pressing and releasing key i causes bit i to become 1

  • Stays that way until software resets it somehow
  • To reset a specific bit, write a 1 into bit i
  • Writing 0s here means don’t change that edge capture bit

• There are 4: KEY0 → KEY3

• They are connected to a parallel port, which is a set of memory-mapped registers

Edge capture register goes to 1 and stays there after release

Example: KEY push buttons with Data Register

• We need two loops, one to check if its been pressed down, the other to see if its been released

• In Lab 4: you must first use data register to detect button push and then release

  • Must make a polling loop to detect the push of key 1

  .equ KEY_BASE, 0xff200050
      movia r8, KEY_BASE
  
  ; polling loop to wait for KEY1 to bepressed 
  poll: 
      ldwio  r9 0(r8) ; loads the data register 
      andi r9, r9 , 0x2 ; bit mask to isolate out the one bit 
      beq r9, r0, poll
      
  ; arrive here only when key 1 is pressed -> bit is 1
  

Example: KEY push buttons with Edge Capture Register

	_start: 
    movia r8,KEY_BASE ; set r8 to base KEY port
    movia r9,LEDs ; set r9 to base of LEDR port
    movi r10, 1 ; first value of LEDR output is 1

poll: 
    ldwio r11, 0xC(r8) ; load edge capture reg
    andi r11,r11,0x4 ; 'select' bit for Key2 is 0100
    beq r11, r0, poll ; if 0, nothing changed -> loop

    stwio r10, (r9) ; turn on/off LED
    xori r10,r10, 1 ; invert r10 for next time

    movi r12, 0x4 ; turn off edge capture bit
    stwio r12, 0xC(r8) ; by writing 1 into bit 2
    ; that takes a lot of getting use to!

    br poll ; go back to poll loop

Hardware Timers

• Used a delay loop to approximate the passage of time in lab 3
• For almost exact time measurement, we need:
  1. A clock signal that is accurate in its frequency/period
     1. Crystal based oscillator
  2. Use a digital counter to count a very specific number of those clock periods
• Nios II DE1-SoC has specific I/O portal to connect to this hardware, so that it can very accurately measure the passage of time

Timer Port Registers in Nios II “interval timer”

Status Register: contains status flags

• Bit 0 TO: Timer Out, which indicates whether the timer has reached the timeout value
  • Becomes 1 when counter hits 0
  • Store 0 to reset