4 ms·

in a way, yes, but those 3 transistors do not affect the simulation. the drains and sources are shorted together and tied to a random data line. the gates are g

in a way, yes, but those 3 transistors do not affect the simulation. the drains and sources are shorted together and tied to a random data line. the gates are grounded. they just sit there, doing nothing.

wyldfire 10y ago
Are they there for some meta feature like spares/patching or part identification, perhaps?
- kevhito 10y ago
  Perhaps they are the circuit equivalent of a trap street [1]? [1]: https://en.wikipedia.org/wiki/Trap_street https://en.wikipedia.org/wiki/Trap_street
pwg 10y ago
With source/drain shorted together and tied to a data line, and the gate grounded, they might just be high resistance pull down resistors. I.e., they would rely on the gate leakage current to create a high ohm resistor. Creating real resistors on silicon IC's is area expensive, so most need for real resistors is first designed around and what's left as absolutely necessary is usually created using tricks. This might be one of those tricks.
- taneq 10y ago
  Knowing nothing about IC or CPU design, this was my first thought. They're there to fix some weirdness that happens when they're not there, and nobody could figure out exactly why it was happening so they just left them there. Kind of like the "magic" switch on the front of the old computer case that shouldn't affect anything, but the computer crashes when it's flipped. :P Hardware is weird.
- adrianratnapala 10y ago
  Very high ohm resistor that. And hopefully that's not it, because the pull-down function might even me needed in a discrete board. BUt I was thinking of gate capacitance, some voodoo impedence matching (or even mismatching) to prevent trouble from some reflection somewhere. In that case, the discrete board can't be expected to have the same set of problems. But even if the transistors played
userbinator 10y ago
It's likely they were left-overs from a previous revision, maybe as part of fixing a different bug. There are signs of this patching in other parts of the chip too, like this one: http://visual6502.org/wiki/index.php?title=6502_datapath_control_timing_fix http://visual6502.org/wiki/index.php?title=6502_datapath_con...

wyldfire 10y ago

Are they there for some meta feature like spares/patching or part identification, perhaps?

kevhito 10y ago
Perhaps they are the circuit equivalent of a trap street [1]? [1]: https://en.wikipedia.org/wiki/Trap_street https://en.wikipedia.org/wiki/Trap_street

pwg 10y ago

With source/drain shorted together and tied to a data line, and the gate grounded, they might just be high resistance pull down resistors. I.e., they would rely on the gate leakage current to create a high ohm resistor. Creating real resistors on silicon IC's is area expensive, so most need for real resistors is first designed around and what's left as absolutely necessary is usually created using tricks. This might be one of those tricks.

taneq 10y ago
Knowing nothing about IC or CPU design, this was my first thought. They're there to fix some weirdness that happens when they're not there, and nobody could figure out exactly why it was happening so they just left them there. Kind of like the "magic" switch on the front of the old computer case that shouldn't affect anything, but the computer crashes when it's flipped. :P Hardware is weird.
adrianratnapala 10y ago
Very high ohm resistor that. And hopefully that's not it, because the pull-down function might even me needed in a discrete board. BUt I was thinking of gate capacitance, some voodoo impedence matching (or even mismatching) to prevent trouble from some reflection somewhere. In that case, the discrete board can't be expected to have the same set of problems. But even if the transistors played

userbinator 10y ago

It's likely they were left-overs from a previous revision, maybe as part of fixing a different bug. There are signs of this patching in other parts of the chip too, like this one: http://visual6502.org/wiki/index.php?title=6502_datapath_control_timing_fix http://visual6502.org/wiki/index.php?title=6502_datapath_con...