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BS2 I/O electrical specs not clear to me — Parallax Forums

BS2 I/O electrical specs not clear to me

FlotulFlotul Posts: 24
edited 2006-03-15 02:12 in General Discussion
Reading the electrical specs for a BS2, I can read:

Source/Sink Current per I/O: 20 mA / 25 mA
Source/Sink Current per unit: 40 mA / 50 mA per 8 I/O

I don't understand:
1) the difference between Source and Sink
2) why it is stated per unit...per 8 I/O

Can someone explain?

Comments

  • Jon WilliamsJon Williams Posts: 6,491
    edited 2006-03-14 16:37
    Source current is when the pin is driven high; Sink current is when the pin driven low.

    There are two pin groups: P0-P7, and P8-P15 -- you'll want to add up all the current requirements for each group and make sure they do not exceed the limit specified.· All of this has to do with the architecture of the underlying silicon.

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    Jon Williams
    Applications Engineer, Parallax
  • FlotulFlotul Posts: 24
    edited 2006-03-14 16:48
    Thank you Jon.

    One more question please.

    If I understand you well, should I connect (i.e.) a 25mA LED's cathode to the BS2 and the anode to Vdd, it would not harm the BS2. Reverse, connecting this same LED's cathode to Vss and the cathode to the BS2, I might have an issue? Is this correct?

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    Roger

    Savigny, SWITZERLAND (french speaking part)
  • Jon WilliamsJon Williams Posts: 6,491
    edited 2006-03-14 17:01
    Yes, that is correct. In practice, I've never found an LED that didn't work quite nicely at 10 mA -- this gives you the ability to set it up as sink or source depending on your coding preference. And if you want to drive a 7-segment display, you'll want to cut the current per LED to 5 mA; still, this is quit visible in all but the harshest lighting conditions (in my experience, anyway).

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    Jon Williams
    Applications Engineer, Parallax
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-03-15 02:12
    Flotul said...(trimmed)
    If I understand you well, should I connect (i.e.) a 25mA LED's cathode to the BS2 and the anode to Vdd, it would not harm the BS2. Reverse, connecting this same LED's cathode to Vss and the cathode to the BS2, I might have an issue? Is this correct?
    I would also recommend putting a resistor in series with the LED.· By default that connection will likely draw more current than you want to.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
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