Category Archives: Hardware

My friend Ken pointed out that unlike a real Elf, my FPGA-Elf didn’t display the contents of memory if you repeatedly press the input button while in load mode with memory protect enabled.  At first I couldn’t figure out how … Continue reading →

The original COSMAC Elf ran at 1 to 2 MHz.  Every machine cycle required eight oscillator cycles, and each instruction required two or three machine cycles, so each instruction took 8 to 12 microseconds at 2 MHz, or 16 to … Continue reading →

Richard Ottosen used my cell phone to shoot a short video of me demonstrating the FPGA-Elf, toggling in and running the dice demo. Thanks to Jim Donnelly, Richard Ottosen, and Jim Phillips for helping me put together the FPGA-Elf front … Continue reading →

I needed a demo program for the FPGA-Elf that was at least a little more interesting than simply counting, so I wrote a dice program.  When you press the input button, it simulates rolling a pair of six-sided dice.  Perfect … Continue reading →

After much gnashing of teeth and pulling of hair, I finally figured out what was causing load mode to fail on the real hardware despite working on the simulator.  My “next state” logic didn’t have a default value, so both … Continue reading →

I’ve added some photos of the completed hardware, showing the Avago hexadecimal displays to my FPGA-Elf set.  I also shot a brief video showing an LED test, though it is not using the 1802 core.  (I got the 1802 core … Continue reading →

I got the switches wired up and working, with a little help from Richard Ottosen.  I tested the debouncer, which worked.  Then I went for broke and loaded the complete Elf system into the FPGA.  After a bit more debugging, … Continue reading →

I’ve wired up the Avago hexadecimal LEDs to the FPGA board and verified that they work with some VHDL test code (not yet tried with the VHDL Elf code). Now I need to wire up the switches. Does anyone have … Continue reading →

I changed the state machine in my Elf code to something much simpler, based on the observation that in load mode the DMA input cycle *always* is acknowledged immediately, and that I don’t have to wait for the 1802 core … Continue reading →

I threw together a testbench for the Elf, with the switch debouncing disabled.  The testbench tries to use load mode to load a program that has an infinite loop setting and clearing the Q flag, which is available as an … Continue reading →