Novel technique using one pin with shift registers

Leon

This might be useful in some situations:

www.romanblack.com/shift1.htm

Leon

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Amateur radio callsign: G1HSM

Beau Schwabe

Leon,

Eexcellent!

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

kwinn

Great idea for when you are desperately short of pins.

Leon

It could probably be implemented using digital delays, if the shift registers were implemented in a CPLD, for people that don't like the analogue technique used.

Leon

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Amateur radio callsign: G1HSM

Phil Pilgrim (PhiPi)

Leon,

That's a great find! I believe his '595 circuit can be improved, however, as follows:



In this circuit, an extended low on the input will cause the RCLK pin to go high, ahead of SCLK going high, which permits the use of all eight data bits. The Schmitt inverter also creates a snap action, which could be important for an edge-triggered input like RCLK.

-Phil

Leon

I also mentioned it on the AVR Freaks forum and someone pointed this out:

www.edn.com/contents/images/6598372.pdf

It's similar to your circuit.

Leon

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Amateur radio callsign: G1HSM

erco

Very nice idea. Out-of-the-box thinking!

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Cluso99

I wonder if this method could also be used for a 1-pin PS2 keyboard driver ???

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Peter Jakacki

It's interesting when you see others using similar ideas. I did a very minimal design years ago for an engine diagnostic display using an 8-pin PIC and a 16x2 LCD. I ran the lcd in 4-bit mode with the R/W tied low (of course) and having only 5 outputs and 1 input on the PIC I used 4 lines for data, 1 line for EN and fed the RS from an RC network hanging off one of the data lines. The one input coupled back to the ECU via a resistor and that was as minimal as you can get I believe (no shift-register).

So Roman's idea to use an RC delay with the shift register is good and practical for those I/O strapped projects and quite useful for driving non-sensitive outputs such as indicators leds.

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*Peter*

Beau Schwabe

Here is a similar approach, published in 1992 as a side magazine to Electronics Now called 'New Ideas'. Unfortunately Electronics Now is no longer being published.

'Electronics Now' Magazine history in case you are interested

On page 13 of 'New Ideas' (attached image below) there is a project called 'Frequency-Boundary Detector' which uses two RC time constants to determine if an input signal is above or below a certain frequency. Sort of like a window comparator, but instead of voltage, it's used for frequency.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.