<emeb>
tnt: modified my 6502 system so it loads the 8kB BASIC from spi flash into SPRAM via the SB_SPI core. RAM has 4kB block-level write-protect so I can make that region act like ROM.
<emeb>
Mostly nice because it frees up BRAMs for other stuff, makes better use of SPRAM.
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<_whitenotifier-1>
[Boneless-CPU] zignig synchronize pull request #3: Text Assembler for boneless (Nearly) - https://git.io/fhxmz
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<mietek>
particularly about the abuse of a mac’s stand-by voltage.
<mietek>
the general idea is to connect USB input devices to ADB macs. if the mac has soft power capability, then it would be nice to let the user press a key combination on their USB keyboard to boot the mac.
<mietek>
this can work when the Teensy is powered from USB, but i also thought it could be possible to abuse the stand-by voltage that is supplied in ADB
<mietek>
this is not a good idea for macs such as II / IIx / IIfx, where the stand-by voltage is battery-backed, hence SW101 for the user to choose if they want this
<mietek>
there’s also SW102 for the user to boot the mac manually
<mietek>
the idea may make more sense if there were more ADB modules — like in a KVM
<mietek>
then, the voltage supplied by any mac could be sufficient to boot every mac
<whitequark>
what is the spec for V.SB on ADB?
<whitequark>
voltage, current...
<mietek>
+5V, current unclear. still need to test it
<tnt>
well ... seems like you short it to GND to power up so must be through a high value resistor.
<whitequark>
yeah
<tnt>
Doesn't seem to be specified at all
<tnt>
all in all, I doubt it would work.
<tnt>
being careful you might be able to run the teensy itself in some low power mode, but then you have the actual input device connected to it that must also need power ...
<tnt>
I mean, assume a 470 ohm pull up (which is ... optimistic). Draw 10 mA from it and ... you're done, you actually have driven the pin to GND and powered up the device.
<mietek>
i measured the amperage in series between V.SB and GND, and it looks like 256 uA max
<mietek>
the measurement resulted in booting the mac
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<tnt>
yup, so 19k pullup or so ... you're not going to power a teensy from that.
<mietek>
yeah, you’re right.
<mietek>
thanks. any other comments about the circuit?
<mietek>
i’m still unsure about values of R0 and C0
<mietek>
that was supposed to decouple the power
<tnt>
the gate resistors look a bit high to me.
<tnt>
R0 and C0 ... I'm pretty sure the teensy has all the decoupling it need onboard. I mean, won't hurt but ...
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<tnt>
I'd go with another diode with lower Vf.
<tnt>
1n5817 in the same series has better Vf for instance.
<mietek>
tnt: did you mean to write 1n5817? that’s the one i’m using