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Getting G3 Whisper Perch USB working

croissantking

Well-known member
I'm trying to work out what oscillator to use, I know for certain it's a 48Mhz part and probably 5V at that. Not sure what SMD package type would fit since the plastic ones don't seem to be made anymore, and the ceramic ones I've been perusing on Mouser are bloody tiny.

Is an oscillator even needed at all? In @zefrenchtoon 's picture of the Yikes! motherboard, there's just an empty pad where that would go; equally, some PCI USB cards have the oscillator while others omit it. I read that the USB chip can generate its own internal clock but it's less reliable than an external crystal.

It is interesting that the Yikes! motherboard employs a pair of poor man's fuses as over-current protection rather than a chip. Were there reliability issues with USB on those machines?

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I have definitely done something screwy with GND. Surely it's not right if there's +5V on the VSS bus, because VSS is a return path, isn't it?
 
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croissantking

Well-known member
I have a new multimeter that can test capacitance, and have been examining the SMD caps on my Belkin donor card. The ones by the USB ports measure 0.5nf whereas most of the others on the card are 100nf. However there are some pink ones that don’t seem to have any capacitance at all, nor continuity. Is there such a thing as dummy capacitors?

Also, could someone please tell me the difference between a ‘pull-up’ and ‘pull-down’ resistor (as these are referenced in the USB chip documentation) and how to tell which is which based on capacitance?
 

mdeverhart

Well-known member
Also, could someone please tell me the difference between a ‘pull-up’ and ‘pull-down’ resistor
A pull-up resistor is connected between a signal and the power rail (say +5V), so that the signal is “pulled up” to a logic 1 if it’s not driven by an active driver, while a pull-down resistor is connected between a signal and Ground so that the signal is “pulled down” to a logic 0 when undriven (this assumes positive logic, where the higher voltage is a 1 and 0V/GND is a 0).

Typical values for pull resistors are 1k - 100k Ohms, though others are possible. A lower resistance is a “stronger” pull, in that the driver must source or sink more current in order to overcome the “pull”, while a high resistance is a “weaker” pull, as less current is required.
 
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croissantking

Well-known member
A pull-up resistor is connected between a signal and the power rail (say +5V), so that the signal is “pulled up” to a logic 1 if it’s not driven by an active driver, while a pull-down resistor is connected between a signal and Ground so that the signal is “pulled down” to a logic 0 when undriven (this assumes positive logic, where the higher voltage is a 1 and 0V/GND is a 0).

Typical values for pull resistors are 1k - 100k Ohms, though others are possible. A lower resistance is a “stronger” pull, in that the driver must source or sink more current in order to overcome the “pull”, while a high resistance is a “weaker” pull, as less current is required.
Thanks!

I think I am going to have to blindly experiment with capacitor and resistor values.
 
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