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IC to control Ground — Parallax Forums

IC to control Ground

edge87edge87 Posts: 24
edited 2010-12-05 10:21 in Propeller 1
I'm having trouble finding an IC that I can use like the 74hc595. What I have is a character block that has a common anode. So to make the segments light up you apply ground to the other 8 segments. I want to be able to control a series of these character blocks without taking up a lot of propeller pins. Has anybody worked on something like this before? The ideal chip would use latching so I could control which segments to light up and it would open up a ground on the ICs pins.

Comments

  • JonnyMacJonnyMac Posts: 9,208
    edited 2010-11-30 08:21
    You could use the 74x595 outputs through a ULN2803 to go from TTL to Open Collector (Darlington). This will bump the current for you, too, something I assume you need.

    I'm using the TPIC6A595 on a project but its inputs are not 3.3v compatible (VIH is > 3.3) so I had to put a buffer in front of it. It has Open Drain outputs capable of 350mA each.
  • edge87edge87 Posts: 24
    edited 2010-11-30 08:34
    Is this the datasheet?

    http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000179.pdf

    If i understand this correctly. inside that IC are transistors. When I apply voltage to the "in" it will turn the "out" to a ground so voltage will flow in and to ground?
  • Duane DegnDuane Degn Posts: 10,588
    edited 2010-12-01 10:25
    edge87,

    You didn't mention if the 74hc595 weren't working for you because of current requirements. I'm not sure if you realize that 74hc595 can both source current and sink current ("1" sources, "0" sinks).

    I haven't used the TPIC6A595. I've used its little brother the TPIC6B595 which has lower current capacity. The A version can sink 350mA while the B is limited to 150mA (each line). I find I can drive the TPIC6B595 directly from the Propeller chip.

    The TPIC6B595 (and I assume the A version) are controlled very similar to 74HC595 chips. The main difference is the TPIC6B595s can only sink current (while the 74HC595s can source and sink). The pins are all switched around with the TPIC6B595s so don't try to use them as drop in replacements.

    I recently used a combination of SN754410 Quad Half H-Bridge (to source current @12V) and the TPIC6B595 (to sink current) to drive six 7-Segment Red 6.5" Displays. All controlled with a Propeller. It makes a cool clock and I also use to display data.

    I haven't tried JonnyMac's Darlington solution yet. A lot of what I've learned about electronics, I've learned from Jon's articles. I recently learned enough PASM to be useful thanks to his SpinZone articles (and reading here on the forum). Thank you Mr. McPhalen.

    Duane
  • edge87edge87 Posts: 24
    edited 2010-12-01 11:21
    Feel kind of silly. I didn't realize that a 8 bit shift register when sent 0's would act as a ground. So if I sent all 1's it would effectively shut off the character display because it has diodes.

    Understanding that I don't think the 74hc595's I have can support the kind of voltage I need to drive the display.(10 volts) ( http://www.mouser.com/Search/ProductDetail.aspx?R=74HC595N,112virtualkey66800000virtualkey771-74HC595N )

    So I'll just supply the 10v to the anode's on the characters, and control the grounding cathode pins with this TPIC6B595. And I should be able to drive the TPIC6B595 the same way I do the 74hc595 , if i understand this correctly.
  • JonnyMacJonnyMac Posts: 9,208
    edited 2010-12-01 11:51
    @Duane: Did you put a buffer between the Propeller and the TPIC6B595? The data sheet says you have to run it at 5v and the *recommended* VIH is 85% VDD which would be 4.25v. To be safe I went with a buffer -- just wondering if that wasn't necessary. My project is an animation controller (designed for a friend) and the outputs drive 12v pneumatic valves and relays, hence I went with the A version for the current.

    Thanks for the kind words about my articles


    @Edge87: Yes, the TPICx595 is compatible with 74x595 code. So it is code-compatible, but not pin-compatible.
  • Duane DegnDuane Degn Posts: 10,588
    edited 2010-12-01 12:22
    Jon, I noticed the .85 Vdd in both datasheets as I was preparing my last post. I double checked my board and I have a wire from a Prop pin to the input pin on the TPIC6B595. When I originally breadboarded the circuit, I used a buffer but I found it wasn't needed so I didn't use one on my final board.

    Duane
  • edge87edge87 Posts: 24
    edited 2010-12-05 10:21
    Would it be possible to explain what the ULN2803A does? I thought I could use it to sink voltage but it doesn't seem to have a way of communicating with it. it either sinks voltage non stop when i have the common free wheeling diodes grounded, or sinks nothing ever when hooked to positive voltage.
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