If you have ever used a system that loads its software from ROM or EPROM, you will know how troublesome it is to frequently change the code. It is not anyone’s idea to pull out the chip, flash it, and then reinsert it into the socket every time the line is changed.This is why [Nick Bild] PicoROM is proposed, which is a method of using Raspberry Pi Pico to simulate ROM chips.
Using Pi Pico instead of the original ROM, it only takes a small amount of time to update the firmware and does not require you to physically disconnect any hardware. [Nick] Similar things have been done with FPGAs in the past, but it is cheaper and easier to use with Pi Pico to make this version particularly attractive. The secret to making it work is the overclocking potential of Pico, which he said has been pushed to 400 MHz for this particular application.
The disadvantage is that you cannot access Pico’s onboard flash memory when the chip is running so fast. To solve this limitation, all codes are loaded into the RAM of the microcontroller.Use healthy 264 KB of memory, which is not really a problem when simulating a 32 KB chip, but [Nick] Said his method will soon fail for larger ROMs.
Except for Pi Pico itself, [Nick] Three 74LVC245AN 8-bit logic level converters are being used, so the chip can communicate with the 5 V logic of his self-made 6502 computer. Everything is connected on a simple breadboard, PicoROM can provide services to the operating system effortlessly because it runs at a frequency of 2 MHz.
Of course, modern high-performance microcontrollers are not absolutely necessary. In the past, we introduced devices that can simulate EPROM using silicon wafers from the 1990s.