3 ms·

Not an EE, but my guess is that current transistors can likely operate at a much higher clock rate than current chips. The problem is that when you pack them t

Not an EE, but my guess is that current transistors can likely operate at a much higher clock rate than current chips. The problem is that when you pack them together, they get far too hot to permit dennard scaling to reach the limits of the individual transistor.

tux3 3y ago
This one's not because of heat, but because signal had to go from the beginning of a stage, through several transistors, to the end of the stage So the transistors switch fast individually, but the speed of the chip is limited by the slowest path in any stage, where you wait for every transistor on the path in series
- mtlmtlmtlmtl 3y ago
  Ah, right, thanks for correcting me.
- foota 3y ago
  Out of curiosity, how long are these paths generally?
  - jiggawatts 3y ago
    Low single digit millimeters for current chip designs. One reason that clock speeds are going above 5 GHz these days is that chips are getting smaller. That means shorter signal propagation distances.
  - tux3 3y ago
    So, when I talked about the length of the path in number of transistors, I oversimplified a bit. The length of the path would be measured in nanoseconds (or even picoseconds). That number depends not just on the number of transistors, but also on the routing delay (are the interconnect wires between transistors long or short? How high is the resistance/capacitance?), and a lot of low-level details of the fabrication process that are extremely not public But say you buy a brand new CPU and it's clocked at 5 GHz, you can easily get a rough estimate of how long the critical path is, since 1/5GHz = 0.2ns What you can't easily get is the speed of the transistors or the number of transistors in the critical path, that info is not public, and you could only make a very rough guesstimate.
- magicalhippo 3y ago
  I recall reading some decades ago about dividing chips into separate domains that would commuicate asynchronously, precisely to shorten the longest path and increase the max frequency. It's my understanding modern processors do indeed do this to varying degrees.
  - tux3 3y ago
    Yep, the term you might be thinking of is "clock domains". This is good if you want different parts of the chip to run at different frequencies, potentially saving a lot of power & gaining perf where needed

tux3 3y ago

This one's not because of heat, but because signal had to go from the beginning of a stage, through several transistors, to the end of the stage So the transistors switch fast individually, but the speed of the chip is limited by the slowest path in any stage, where you wait for every transistor on the path in series

mtlmtlmtlmtl 3y ago
Ah, right, thanks for correcting me.
foota 3y ago
Out of curiosity, how long are these paths generally?
- jiggawatts 3y ago
  Low single digit millimeters for current chip designs. One reason that clock speeds are going above 5 GHz these days is that chips are getting smaller. That means shorter signal propagation distances.
- tux3 3y ago
  So, when I talked about the length of the path in number of transistors, I oversimplified a bit. The length of the path would be measured in nanoseconds (or even picoseconds). That number depends not just on the number of transistors, but also on the routing delay (are the interconnect wires between transistors long or short? How high is the resistance/capacitance?), and a lot of low-level details of the fabrication process that are extremely not public But say you buy a brand new CPU and it's clocked at 5 GHz, you can easily get a rough estimate of how long the critical path is, since 1/5GHz = 0.2ns What you can't easily get is the speed of the transistors or the number of transistors in the critical path, that info is not public, and you could only make a very rough guesstimate.
magicalhippo 3y ago
I recall reading some decades ago about dividing chips into separate domains that would commuicate asynchronously, precisely to shorten the longest path and increase the max frequency. It's my understanding modern processors do indeed do this to varying degrees.
- tux3 3y ago
  Yep, the term you might be thinking of is "clock domains". This is good if you want different parts of the chip to run at different frequencies, potentially saving a lot of power & gaining perf where needed