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PostPosted: Sun Mar 14, 2021 2:39 pm 
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Joined: Tue Jul 07, 2020 10:35 am
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Location: Amsterdam, NL
I have what is marked as an R65C51P2. I currently have a 1.8432Mhz oscillator can hooked up to pin 7, XTALO. This has been working. But when I was drafting a small PCB to make the circuit permanent, I rechecked which pin this should be and all the literature says to use pin 6, XTALI. When I switch my wire to this pin, nothing works anymore! Is there any reasonable explanation for this discrepancy?


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PostPosted: Sun Mar 14, 2021 3:56 pm 
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Joined: Fri Aug 30, 2002 1:09 am
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I can't imagine it working if you put the oscillator to XTAL-out! It has to go to pin 6. I've never had any trouble with it going to pin 6. Always worked as intended, from the beginning.

Edit: On my workbench computer, I have one '51 with just a crystal across it as floobydust below says (not an external oscillator), and this ACIA's output can optionally drive a couple others' inputs, on their pin 6.

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PostPosted: Sun Mar 14, 2021 6:04 pm 
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I've always used a crystal with the 6551 (and 65C51) which works fine. By driving the output with a can oscillator, I suppose there's an outside chance that the internal inverter (or whatever drives the output) got fried. The simplest test would be to try a crystal and the 20pf cap from the input to ground. Note: some 65C51 chips might require a 1meg resistor in parallel with the crystal.

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PostPosted: Sun Mar 14, 2021 8:28 pm 
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Procrastin8 wrote:
I have what is marked as an R65C51P2. I currently have a 1.8432Mhz oscillator can hooked up to pin 7, XTALO. This has been working. But when I was drafting a small PCB to make the circuit permanent, I rechecked which pin this should be and all the literature says to use pin 6, XTALI. When I switch my wire to this pin, nothing works anymore! Is there any reasonable explanation for this discrepancy?

To quote the WDC data sheet:


    Crystal Pins (XTLI, XTLO)

    These pins are normally directly connected to the external crystal (1.8432 MHz) to derive the various baud rates. Alternatively, an externally generated clock can drive the XTLI pin, in which case the XTLO pin must float. XTLI is the input pin for the transmit clock.

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PostPosted: Mon Mar 15, 2021 9:04 am 
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Joined: Tue Jul 07, 2020 10:35 am
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Location: Amsterdam, NL
I swapped out the current chip for an identical one and pin 6 works on it, so the theory I might have burnt out the first seems possible, though strange that it would accept the input on 7 afterwards. Thank everyone!


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