This circuit started out with the need for a long shift register. I wanted to make a digital delayline, using the bit-serial encoders as described in the US-patent #4462106. The circuit shown is just the delay core, and part of a larger system. To make it work with the delta encoders or similar ones, one needs to do some levelshifting.
Although one can make such a circuit from standard MSI parts, I opted for a uC based design. My choice fell on the AT90S1200 since they're pretty fast compared to a PIC. It lacks most of the fancy peripherals and the RAM that its bigger brothers (and sisters) have, but I didn't want to make use of them anyway. Anyway it should be straight forward to port it to another ATMEL device. The software is just a pretty short loop, that asserts the R/W' CAS' and RAS' lines in the proper order, and implements an 18-bit address counter that is multiplexed onto the 9 address lines of the DRAM. The 256k types where just what I have here in quantity, and if you have some old 286 or even 8088 based computer that you use as a door stopper, then you'll find plenty of them inside. I've also made a variation that uses 64k DRAMs. These will be connected only to the lines A0-A7. Also one could hack something out to use 1Mbit RAMs. The operation is quite simple, the input of the DRAM appears 256k R/W' cycles later at the output.
It's possible to extend the amount of RAM, by bussing the address lines and the R/W' CAS' and RAS' lines out to several DRAMs. You can either use them in parallel with some parallel ADC / DAC or chain them all in series for a bit serial encoder. In the latter case you simply tie the Din pin to the Dout of the preceeding DRAM.
To get hold of a clocking signal at the actual sampling rate (which is 1/9th of the uC clocking rate) you can make use of the R/W' line. I have had to shift that signal to send it to the encoders. (A level translator chip is handy here.) The clock for the uC is generated in a VCO of a 74HC4046 which can go up to 12MHz. I have the idea of makeing that into a PLL so that I can steer the delay time with trigger pulses, for easy interfacing with sequencers. When I get arround doing that I'll place the schematic here. Block level view of the entire delay.
What would a uC be without its software? Here are the files:
Assembler file 256k Version
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Intel Hexfile 256k Version
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Assembler file 64k Version
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Intel Hexfile 64k Version
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I think there are many other uses for this circuit, like a trigger/pattern memory, if you feed the output back into the input via a resistor. To program you then supply your pulses with a low impedance to the input, and disconnect this lower impedance for the feedback to take over. It might also be interesting to interface a serial ADC/DAC to this circuit. Although the software would need modifications, and send some other control signals to the ADC/DAC. One then probably needs a refresh logic since one wouldn't need to clock the thing so high as I did. But the software could handle this.
