CS452 F23 Lecture Notes
Lecture 10 - 17 Oct 2023

• e.g., poll track sensor

naive approach

  loop:
    action();
    Delay(tid, X);
  endloop;

better:

  target = Time(tid);
  loop:
    action();
    target += X;
    DelayUntil(tid, target);
  endloop;

• TX (transmit) - when there is sufficient room in the transmit FIFO
• RX (receive) - where there is sufficient data in the receive FIFO
• CTS - when the CTS flow control signal changes state
• and more!

• one bit per interrupt: 1 enables interrupt, 0 disables it

• logical OR of all interrupt signals from all UARTS

• set appropriate bit(s) in IMSC
• enable Interrupt ID in the GIC, and set CPU target (like for timer interrupt)

• read GICC_IAR to find the InterruptID
• if it is a UART interrupt
  • read UART RIS or MIS register(s) to find out which UART interrupt(s)
• clear interrupt using UART ICR (and eliminate cause of interrupt if necessary)
• write to GICC_EOIR when finished, as for the timer interrupt

• allows for small bursts of writes
• but can’t control when queued writes are sent
  • mostly a problem for Marklin, not Console
    • Marklin needs to finish processing previous request before the next request can be sent
  • in theory hardware flow control should prevent sending too fast
    • in practice, Marklin might not de-assert CTS fast enough!

• helps buffer bursts of incoming data
• but can’t get an RX interrupt on non-empty RX FIFO
  • interrupt “trigger levels” are 1/8 full, 1/4 full, etc. (see UART IFLS register)

• UART Tx interrupt generated when output buffer is empty
  • but this only tells you that a command has been sent (not processed)
• UART Rx interrutp generated when input buffer is full

• how long to wait? Conservative estimate…
• waiting ==> spinning

• pattern:
  • send command
  • wait for CTS de-assert
  • wait for CTS re-assert
• States: Ready -> CommandSent -> DeviceBusy -> Ready

• unsigned char Getc(int tid, int channel)
• int Putc(int tid, int channel, unsigned char ch)

• when does Putc return, does it block
• does Getc block?