419 DJ8 8-bit CPU

419 : DJ8 8-bit CPU

  • Author: DaveX
  • Description: DJ8 8-bit CPU with parallel Flash / RAM interface
  • GitHub repository
  • Clock: 14000000 Hz

How it works

DJ8 is a 8-bit CPU, originally developped for XCS10XL featuring:

  • 8 x 8-bit register file
  • 3-4 cycles per instruction
  • 15-bit address bus
  • 8-bit data bus

Memory Map

From To Description
0x0000 0x7fff External memory
0x8000 0xffff Internal Test ROM (256 bytes, mirrored)
External memory map if using the recommended setup (see pinout)
From To Description
0x2000 0x3fff External RAM (32 bytes)
0x4000 0x5fff External Flash ROM (16KB)

Pinout

Due to TT06 IO constraints, pins are shared between Address bus LSB and Data bus OUT. It means that during memory write instructions, the address space is only 64 bytes.

Pins Standard mode During memory write execute+writeback cycles
ui[7..0] Data bus IN Data bus IN
uio[7..0] Address bus LSB (7..0) Data bus OUT
uo[6..0] Address bus MSB (14..8) Address bus MSB (14..8)
uo[7] Write Enable Write Enable

You can connect a 8KB parallel Flash ROM + 32b SRAM without external logic and use uo[6] for RAM OE# and uo[5] for Flash ROM OE#.

To get a bidirectional data bus (needed for SRAM), uio bus must be connected to ui bus with resistors. To be tested!

Reset

At reset time, PC is set to 0x4000.

All other registers are set to 0x80.

How to test

An internal test ROM with two demos is included for easy testing. Just select the corresponding DIP switches at reset time to start the demo (technically, a jmp GH instruction will be seen on the data bus thanks to the DIP switches values, with GH=0x8080 at reset).

Demo 1: Rotating LED indicator

SW1 SW2 SW3 SW4 SW5 SW6 SW7 SW8
0 0 0 0 0 0 1 0

No external hardware needed. This demo shows a rotating indicator on the 7-segment display. Its speed can be changed with DIP switches, the internal delay loop is entirely deactivated when all switches are reset.

Demo 2: Bytebeat Synthetizer

SW1 SW2 SW3 SW4 SW5 SW6 SW7 SW8
0 0 0 0 0 1 1 0

Modem handshakes sound like music to your hears? It's your lucky day! Become a DJ thanks to 256 lo-fi glitchy settings.

Connect a speaker to uo[4] or use Tiny Tapeout Simon Says PMOD.

It is highly recommended to add a simple low-pass RC filter on the speaker line to filter out the buzzing 8kHz carrier. Ideal cut-off frequency between 3kHz and 8kHz, TBD.

Set SW1 and/or SW2 at reset time to adjust speed in case the design doesn't run at 14MHz.

External hardware

  • No external hardware needed for demos
  • Otherwise: Parallel Flash ROM + optional SRAM

IO

#InputOutputBidirectional
0data in 0address out 8address out 0 / data out 0
1data in 1address out 9address out 1 / data out 1
2data in 2address out 10address out 2 / data out 2
3data in 3address out 11address out 3 / data out 3
4data in 4address out 12address out 4 / data out 4
5data in 5address out 13address out 5 / data out 5
6data in 6address out 14address out 6 / data out 6
7data in 7write enableaddress out 7 / data out 7

Chip location

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