Suggestions for a 4-BIT SHIFT REGISTER for BDC thumbwheel?

DavidM

Hi,

I have successfully used a 74HC165 8-BIT shift register to read the output values from two BCD thumb-wheels, This works fine for just two thumb-wheels..

But, I may need to use 3 or 4 thumbwheels in other designs.

Currently my circuit requires two sets of resistors ( 8 all up) for Hi & LOW states for each thumb-wheel.

I want to make a small PCB that attaches to the back of the BCD thumb-wheel(s), so I need to keep my circuit board small and it also needs to be through-hole.

So , Is there a different shift register the has the following features ..

1) Has only 4 parallel inputs ( i.e 4 BIT ) ( the only ones I have seen have 4 outputs as well)
2) Does not require resistors ( i want to simply wire the 4 inputs of the register directly to BCD THUMBWHEEL and the common of the thumbwheel back to a common on the chip.
3) they need to be cascade-able ( just like the 74HC165 )
4) has serial output so I can use my same SHIFT IN code as for the 74HC165

any suggestions?

regards

David M

Paul Baker

Take a look at the 74HC194, it is the closest to your list of features.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

DavidM

Thanks Paul,

But thats one of the chips I had already looked at,

Can the thumb-wheel be connected direct to the IC without resistors ?

regards

Dave M

Paul Baker

I think the pull-up/down resistors are required by the electrical nature of the thumbswitch, I am not aware of any shift register which has internal strapping resistors.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Peter Jakacki

Hi Dave, have you considered PCF8574 8-bit I2C port expander which uses weak pullups internally. There is also the PCF8575 16-bit expander as well. I used to use this sort of device around 20 years ago but they are still around and active. You can connect them up to the Prop's I2C bus for the program EEPROM without any problems and save a few lines.

focus.ti.com/docs/prod/folders/print/pcf8574.html

I am surprised though that you are using thumbwheel switches as they are a bit expensive and sort of "old tech" but I suppose do have their uses as they do not require a display.

*Peter*

DavidM

Hi peter,

How about this chip then, PCA9536 its a 4BIT version! and only 8 pins!,

Can someone confirm then, if the BCD outputs can be directly connected to the inputs of this chip (using PINS PO-P3) without resistors?

Thanks

Dave M

Graham Stabler

The datasheet says it has internal pullups on the front page so it sounds like it.

Graham

Mike Green

The BCD outputs could be directly connected to the PCA9536, but you can't have more than one of these on an I2C bus. The PCA9554 is a similar 8 I/O device in that it has weak pullups on the I/O lines. The BCD common line can be connected to ground for this. You could have one or two thumbwheel switches per device and you can connect 8 addressable devices to one I2C bus. There's another device (PCA9554A) which is the same except for a different address range. This would let you connect up to 16 devices to the same pair of I/O pins.