648 JA Achtung Compact
648 : JA Achtung Compact
- Author: Joel Kaplan and Amit Elmaliach
- Description: Two-player Achtung die Kurve: VGA output, QSPI PSRAM framebuffer, gamepad input
- GitHub repository
- Open in 3D viewer
- Open in Tiny Tapeout Explorer
- Open in VGA Playground
- Clock: 50000000 Hz
How it works
JA Achtung Compact is a two-player "Achtung, die Kurve!" style game on a single Tiny Tapeout tile. Each player steers a moving head that leaves a permanent trail; touching a wall or any trail ends the round.
The chip has no internal framebuffer. Instead it uses an external QSPI PSRAM (APS6404L) as a 1-bit-per-cell occupancy framebuffer and generates 640x480@60 VGA directly:
- During active video, a display streamer fetches 4-byte bursts from the PSRAM and expands each stored bit to 4 VGA pixels.
- During vertical blanking, the game engine gets the PSRAM bus: it advances each player on a 4x4-pixel grid, reads the target cell for collision detection, and writes the new trail cell back (read-modify-write, one byte at a time).
- A small arbiter multiplexes the two clients onto one QSPI PSRAM controller (25 MHz SCLK from the 50 MHz system clock).
Player input comes from the Tiny Tapeout Gamepad PMOD (two SNES-style controllers on one serial interface). A compact decoder extracts only the buttons the game needs: L/R shoulder buttons turn each player 90 degrees (edge-triggered), and Start restarts the round after a game over. Player 2's trail is drawn with a woven "groove" sub-pattern so the two monochrome trails are distinguishable.
How to test
- Connect the VGA PMOD, QSPI PSRAM PMOD, and Gamepad PMOD (pinout below).
- Assert
rst_nlow before powering the PSRAM. After PSRAM VDD is stable, keeprst_nlow for at least 150 us. During this hold the chip keeps the external PSRAM clock low, all chip-select outputs high, and all four SIO lines actively low, as required by the APS6404L power-up specification. The 50 MHz system clock may already be running. - Release reset. The chip issues the PSRAM reset and quad-enable sequence, waits the required reset recovery time, clears the framebuffer, and starts the round.
- Player 1 turns with L/R on controller 1, player 2 with L/R on controller 2. After a crash, press Start on either controller for a new round.
In simulation, the full verification suite lives in Game Simulation/tests/:
game-logic traces compared bit-exactly against a Python reference model, a
pixel-exact VGA capture test through the real QSPI read path, and a
full-system smoke test through the gamepad protocol. See
Game Simulation/README.md.
External hardware
- TT VGA PMOD on the dedicated outputs
- TT QSPI PSRAM PMOD (APS6404L) on the bidirectional pins
- TT Gamepad PMOD with one or two controllers on inputs 4-6
- VGA monitor
IO
| # | Input | Output | Bidirectional |
|---|---|---|---|
| 0 | VGA R1 | Flash CS# (unused, high) | |
| 1 | VGA G1 | QSPI PSRAM IO0 | |
| 2 | VGA B1 | QSPI PSRAM IO1 | |
| 3 | VGA VSync | QSPI PSRAM SCK | |
| 4 | Gamepad PMOD latch | VGA R0 | QSPI PSRAM IO2 |
| 5 | Gamepad PMOD clock | VGA G0 | QSPI PSRAM IO3 |
| 6 | Gamepad PMOD data | VGA B0 | QSPI PSRAM CS# |
| 7 | VGA HSync | PSRAM B CS# (unused, high) |