8255 or 8155 Timer / PIO IC's

T&E Engineer · 2005-10-09 20:19

Has anyone tried to interface either of these old IC's with the BS2 stamp?

I remember experimenting with the 8255 24 I/O PIO IC in college.

However, I saw the 8155 in a local electronics store and got it confused with the 8255.

Any info / comments would be appreciated.

Chris Savage · 2005-10-09 22:29

I used the 8255 PIA back in my Z80 days, but never thought to try and use it on the BASIC Stamp since shift registers and other I/O expanders are a better solution.

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Chris Savage
Parallax Tech Support
csavage@parallax.com

T&E Engineer · 2005-10-10 20:10

I have the 8155 and just wondering if I should return it. I have a few 74HC595's as well that might be used in replacement of the 8255? Is the 8255 worth buying (what advantages does the 74HC595 have over the 8255 PIA)?

Application is an LED moving display sign. The 8255 PIA has 24 outputs and I need to have 20 for my (7 x 20 LED display I made). Also I may have wired up my display backwards in that I wanted to use the 74HC595 (or 8255 PIA) for the 20 Cathode leads (-) from the 7x20 LED array. However, I beleive the 74HC595 requires Anode leads (+) not Cathode (-). If this is the case then I may have to use 74HC04 Inverters unfortunately. Is this true?

Comments welcome and appreciated.

Chris Savage · 2005-10-10 23:16

Okay,

···The advantages of the 8255 (I can't speak for the 8155) over the 74HC595 is that the 8255 is programmable for inputs and outputs.· The 74HC595 is simply an output device.· However, you can daisy chain them and get 24 outputs from the same 3 I/O pins and use one line of code to get the data out to them.· See the Binary Digital Clock Project in the Projects forum for an example.· In that clock I have three 74HC595s all shifting out data to the binary LED display.·

·· In this case because you only need outputs, the 74HC595 is not only cheaper, but simpler to use, and I believe can handle more current.· Besides, if you need inputs, the complement to the 74HC595 is the 74HC165.· Using these it is possible to get 32 inputs and 32 outputs from 4 I/O pins on the BASIC Stamp using 4 bytes of variable space.

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Chris Savage
Parallax Tech Support
csavage@parallax.com

Post Edited (Chris Savage (Parallax)) : 10/10/2005 11:20:31 PM GMT

Chris Savage · 2005-10-10 23:26

tdg8934 said...(trimmed)
However, I beleive the 74HC595 requires Anode leads (+) not Cathode (-). If this is the case then I may have to use 74HC04 Inverters unfortunately. Is this true?

·· Just realized I missed that last question.· The answer is no, it's not true.· The 74HC595 is simply going to have a 1 or 0 where you put them on the outputs.· This corresponds to a 5V or 0V at the output.· Worst case scenario is you have to invert the data you are sending out to the shift registers.· This is easily done and wouldn't require a hardware change.· Instead you would output a HIGH to turn off the LED instead of the other way around.·

·· Interestingly this can appear to make the code more complicated as some people might say I'd rather change the hardware than have to think backward, but you don't.· Treat the code the normal way you do, and invert it in the SHIFTOUT.· It will be a transparent change and be functionally equivalent.

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Chris Savage
Parallax Tech Support
csavage@parallax.com

T&E Engineer · 2005-10-11 01:33

Chris,

Thanks so much for the information. I will not have to use the inverters now. That's great.

I am fairly new to this stuff and really love it. Check out my first project posted using the DS1302 and SX Video module I'm getting ready to post on the forum in·a few minutes.