I need a "hint" with interfacing Thumbwheels.

G McMurry

I need to decode 30+ addresses with a Thumbwheel switch. I have a pair of BCD thumbwheels, 0-9 X 2. I want to use the least number of STAMP pins for the task.

Doing it with 8 bits is a no brainer --
Doing it with 6 is also easy. Each thumbwheel 1,2,4,8 through a diode, then a separate wire to each common. 4 + 2 = 6.

I have also been looking at building a resistor network and using RCTIME to decode the setting. Is this going to be reliable?

Does anyone have any other ideas?

On the system have, I easily have 4 bits on a connector. I don't mind building some external circuitry, itf I use the 6-bit technique above, I am already adding diodes.

I would love to get some ideas from this forum.

Greg

Sapphire

If external circuitry are okay, I'd use a 74HC165 to take in all 8 lines and read them with a SHIFTIN command. You'll only need 3 pins, and if you have more inputs later, you can cascade them.

G McMurry

I am looking at that. I think I can handle using 3 pins.

I have been also looking at making a resistor ladder between all the switch contacts and decoding it with a single connection using RCTIME. Now the whole idea is sort of an adventure.

If I use RCTIME, I might have to use quite a bit of memory for a lookup table. Maybe not worth the trouble.

Greg

G McMurry

If external circuitry are okay, I'd use a 74HC165 to take in all 8 lines and read them with a SHIFTIN command. You'll only need 3 pins, and if you have more inputs later, you can cascade them.

OK -- This is how I am doing it.  Besides all the wiring mistakes I made (see picture) I now have a working BDC Thumbwheel interface. 

So now I have two nibbles representing the two 0-9 thumbwheel switches.  How do i get them to come out as 0-99 in a byte? 

I had a socket on my board for an EEPROM that I never needed.  It was easy to wire that socket to a 74hc165 and two decimal thumbwheel switches with pullups.  Now I need to take the two nibbles and turn them into a single decimal value I can use in my software.

Greg

G McMurry

I am struggling with the new forum a bit and couldn't figure out how to attach another image to my original post using the edit function.  It would only let me provide an image as a link.

I have attached my schematic for those who might want to do something like this.

The code I used to acquire the data is right out of the "book" except that I used the /Q output of the 74HC165 to invert the Thumbwheel data.

Get_Thumb:  'Use the EEPROM Socket to connect 74HC165 to read two Decimal Thumbwheels
  PULSOUT EEPromCS, 5                                                   ' grab the switch inputs
  SHIFTIN EEPromDataIO, IOClkPin, MSBPRE, [Thumbs]  ' shift them into variable Thumbs
RETURN

Works fine, I just would like to turn the 2 nibble byte into one bye that represents 0-99.  I could use some advice.

Thanks

Sapphire

Use this:

Thumbs = 10*Thumbs.NIB1 + Thumbs.NIB0

G McMurry

That looks easy enough...  THANKS!

G McMurry

Here is what I ended up with...  Works great!

Get_Thumb:  'Use the EEPROM Socket to connect to 74HC165 to read two Decimal Thumbwheels
  PULSOUT EEPromCS, 10                               ' grab the switch inputs
  SHIFTIN EEPromDataIO, IOClkPin, MSBPRE, [Thumbs]  ' shift them in

  Thumbs = (Thumbs.HIGHNIB * 10) + (Thumbs.LOWNIB)  'now combine the 2 thumbwheels to read 0-99
  Thumbs = Thumbs MAX 28                            'limit the Thumbs value to 28
RETURN

Thanks for all the help on this one.

Greg

Sapphire

Great. You can even put the MAX 28 on the same line:


[code]

G McMurry

Thanks for that...  I like things that combine commands on a single line.

Greg