CS 452/652 Spring 2025 - Lecture 2

Devices and Trains

May 8, 2025 prev next

BCM - Broadcom BCM2711 data sheet

Memory

• see BCM Section 1.2.1, Figure 1 “Low Peripheral”
• main memory: assume 0x00000000 - 0x3FFFFFFF (1 GB)
• top 76MB used for VC (config.txt)
• memory-mapped device registers: "magic" addresses
  • see BCM Section 1.2.4: base address 0x7E000000 → 0xFE000000

Timer

• no time of day needed, no remote synchronization (very hard problem)
• ARM timer (BCM Chap 12) vs. System timer (BCM Chap 10)
• ARM timer follows CPU-frequency → use system timer
• system timer
  • free-running counter at stable frequency: 1 MHz
  • 7 control registers starting at offset 0x3000
  • CLo/CHi -> timer counter
  • C0-3: compare and set status in CS
  • query timer and/or wait for interrupt via compare registers

General-Purpose Input/Output (GPIO)

• configurable hardware interface
• BCM Chap 5, Page 78, registers at offset 0x00200000
• iotest contains code for setting up GPIO
• no reason to mess with this

Universal Asynchronous Receiver/Transmitter (UART)

• serial (character) device: 1 byte at a time
• UART: standard "state machine" at the end of serial line
• BCM Chap 11 (not Chap 2): UART0 and UART3
  • device registers at offset 0x201000 (UART0) and 0x201600 (UART3)
  • data register (DR) at offset 0x00
  • FIFO buffer (optional, default size 1)
• configuration: speed, byte representation, FIFO
  • Channel 1 (terminal): 115200 baud (8 bits, no parity, 1 stop bit)
  • Channel 2 (train): 2400 baud (8 bits, no parity, 2 stop bits)
• output (transmit, TX)
  • write to DR
  • UART moves to FIFO
  • FIFO entries sent via serial line
• input (receive, RX)
  • reception from serial line into FIFO
  • first element in FIFO is always available in DR
  • read from DR
• flag register (FR) describes status of TX/RX buffers (full/empty)
  • check before send/receive
• timing
  • actual communication happens in parallel to UART processing
  • receive: need to be fast enough
  • transmit: paced according to line speed when checking TX state in FR
  • how long does it take to transmit a byte to the console? to the Marklin controller?
  • sender speed vs. serial line speed vs. receiver speed
    • later: flow control support
    • for A0: manually wait (~20ms for train) after each train command

Trains & Track

• 1- or 2-byte commands: operation (, address)
• trains:
  • 0-14 speed level
  • 15 reverse direction
  • add 16 to turn on headlights
• trains slow down gradually
  • do not directly reverse a rolling train
• switches:
  • 0x21 straight, 0x22 turn (curved)
  • changes a switch setting actives a solenoid
    • deactivate at most 250ms after sending the switch command
    • deactivate by:
      • sending another switch command (to any switch)
      • send the special solenoid off command (0x20)
        • single byte - turns off solenoid from last switch
  • middle "double" switches
    • avoid C/C state (and S/S is not very helpful either)
    • if curved left (CS), and want curved right (SC)
      • first set C switch to S (SS)
      • then set other switch to C (SC)
• sensors:
  • 5 banks of 16 up to 16 sensors
    • banks labeled as A, B, C, D, E on track
    • individual sensors labeled A13, A14
    • sensors are directional: A15 →, A16 ←
    • when trains trigger sensor, changes sensor state in Marklin controller from 0 to 1
  • read the state of all sensors by sending command 0x85 (0x80 + 0x05)
    • means read state of all banks up to and including 5th banks
    • then read 10 bytes of data from the Marklin
      • two bytes (16 bits) per bank
  • or read individual banks (0xC0 + bank number)
  • controller is in one of two sensor reset modes: on or off
    • if reset mode is on, the sensor bits are cleared each time they are read
    • if reset mode is off, reading the sensor bits does not clear them
  • The single-byte command 0xC0 can be used to turn the reset mode to on (0x80 turns off).

Notes

• turn on the headlights! (verify communication and direction)
• can use extra serial cable to connect gtkterm to RPi Channel 2 for debugging
• IMPORTANT: turn off the last solenoid using 0x20!
• IMPORTANT: track communication is half-duplex, i.e., no control during sensor reporting

Polling Loop

• polling ≠ busy-wait

  for (;;) {
    if (c1) a1();
    if (c2) a2();
    ...
  }
  

  vs.

  for (;;) {
    while (!c1);
    a1();
    while (!c2);
    a2();
    ...
  }