=========================
Gigatron startup sequence
=========================

ROMv1..v4

Hardware start
--------------

1. Power on
   - The blinkenlight LEDs are in an arbitrary state.
     This might be 4 lights on, or 4 lights off, or
     some on and some off.

2. The MCP resets the program counter to 0 and holds it
   - The Power OK light will be "off"
   - The system is repeatedly executing the first instruction,
     because the clock circuit will startup anyway
   - The blinkenlight LEDs hold their initial arbitrary state

3. Wait until supply voltage is stable above 4.5V
   - Some of the early kits wait for 4.75V because that was
     needed for the original 74LS chips. Unfortunately this
     requires a good combination of power supply and power
     cable. The 74HCT chips can work with a lower voltage so
     we changed this to 4.5V (MCP450 instead of MCP475).
   - When the voltage is above 4.5V for one third of a
     second, the MCP releases the reset line
   - With that the "Power OK" lights up and the program
     counter is allowed to run

Software start
--------------

4. Switch all 4 blinkenlight LEDs off

5. Reset the I/O and RAM expander in case it is present
   - Not in ROMv1..3
   - Enable RAM (it might be disabled after power on)
   - Set bank to 1
   - Disable any SPI devices

6. Do the quick RAM check
   - Counts the available memory (32K or 64K)
   - Presence of 128K isn't detected

7. Perform 10 ms delay loop
   - This was originally for debouncing the reset button
     on the breadboard version. The PCB kit version
     doesn't have a hardware reset button, and the memory
     scan that follows has the same effect anyway. We keep
     it for historic reasons.

8. Switch left-most LED on

9. Full memory scan
   - This collects entropy from the uninitialized memory

10. Switch second LED on

11. Test for cold or hot boot condition
    - Only in ROMv1..3

12. Setup vReset routine at RAM location 496

13. Setup serial input

14. Switch third LED on

15. Setup the sound tables
    - Only in ROMv1..3

16. Switch last and right-most LED on

17. Start video signal generation, including:
    - Waveform synthesis for sound channels. This is
      always active, even when the sound is off.
    - Checking the input port 60 times per second
    - Back-and-forth scanning LED pattern
    - Updating the entropy bytes (addresses 6..8)
      This is influenced by input events (value and time)
    - From ROMv2 onwards, the initial video in MODE 3
      for faster startup (3 black lines per pixel line)

18. Load precompiled Reset.gcl from ROM and execute
    - This clears the screen
    - Then writes the top two lines:
        +--------------------------+
        | *** Gigatron 32K RAM *** |
        | TTL microcomputer ROM v4 |
        |                          |
    - The detected memory size is reported on the first line
    - In ROMv4 this also sets up the sound tables
    - The sound channels are set with a G-major chord used
      as the startup chime. But the sound isn't played yet.
    - Change to MODE 1 (one black scanline per pixel line)

19. Load and execute MainMenu.gcl as Main
    - Easter Egg activation is detected
    - Four sound channels are enabled
    - The startup chime is played
    - The menu is shown

20. Wait for user input while flashing the arrow

-- End of document --