Maker Rue Mohr has determined to therapeutic massage some {hardware} into doing one thing uncommon, making a “self-clocking ROM” able to appearing like a crystal oscillator — after a trend, at the very least.
“There are a number of initiatives that may be carried out with a ROM and want a clock supply,” Mohr explains. “So, I used to be within the bathe and had an concept. The thought was so as to add an LC [inductor-capacitor] delay between an information output line and an handle enter line of a reminiscence chip as a way to create the inverter wanted for constructing an oscillator. Usually I’d reject this concept, writing off that that system would discover a voltage between logical 1 and 0 that it could stabilize on, however I would lately been studying RAM datasheets that talked about that they had Schmitt triggers on all of the inputs…”
In concept, then, it ought to be doable to show the ROM chip itself into an oscillator — making it its personal clock supply and getting rid of the necessity for an exterior crystal. To show the idea, Mohr set about constructing a seven-segment show system — however first there would must be some experimentation to see if it was even doable.
“I constructed a ROM picture for a easy inversion. D0 could be the alternative of A0. I wrote a 2816 (16k EEPROM) to do the take a look at,” Mohr explains. “I are likely to assemble circuits with the facility on. To my shock, this began oscillating earlier than I used to be even end constructing it! [But] simply because one thing works, doesn’t suggest it really works properly. It may be essential to play with it and perceive the bounds to operation.”
The venture began with an LC circuit earlier than switching to an RC circuit for cleaner operation as much as 3MHz. (📷: Rue Mohr)
Additional investigation revealed that switching from the LC delay circuit to an RC, resistor-capacitor, delay circuit not solely continued to work however provided improved efficiency — permitting the clock to hit 3MHz. As soon as it got here to really utilizing the clock supply, although, issues turned trickier: “Loading the ‘oscillator’ like this induced a big error within the obligation,” Mohr explains. “Obligation error in one thing like this causes one show to be brighter than the opposite, so I added a number of bias elements to deliver it nearer to 50 per cent.
“At 50 per cent (should you regulate it such) the operation of the oscillator is just not steady, it could stall or not begin up with energy. [It] serves me [right] for not including one other chip (74xx541) to drive the LEDs — its not good type to make a chip like this EEPROM attempt to drive LEDs. Nevertheless it was all OK, and kinda simply labored!”
Extra particulars, together with supply code and binary ROM pictures, can be found on Mohr’s Hackaday.io venture web page.
