5 ms·
I am not sure what you mean by "just an electrical current". The CPU is typically kept in reset until the clock is stable, so it has a valid clock signal on st
by deepspace 3y ago
I am not sure what you mean by "just an electrical current". The CPU is typically kept in reset until the clock is stable, so it has a valid clock signal on startup. It is therefore able to start executing the microcode which performs the power-on-reset sequence, i.e. start reading instructions from the reset vector and so on.
- MichaelZuo 3y ago> CPU is typically kept in reset until the clock is stable > It is therefore able to start executing the microcode At the very beginning, what is keeping it in 'reset' and initiating the execution of the microcode? Through what means? From what I understand, in the first few hundred nanoseconds it's just an electrical current that's flowing through the CPU and nothing else.
- convolvatron 3y agoyeah, ok, reset would normally be held low - whether that's the presence or absence of a current isn't important. in the the old days there was usually a hardware power controller that doesn't raise reset until the power is stable. on those machines I think that just took long enough that the clock chain had settled out. these days its more likely that there is a system controller orchestrating the bringup. potentially waiting for all the voltage converters and the clock generator to settle before raising the reset line. depending on the architecture it may not really be a simple pin, but addressed by the system board controller through the scan network.
- MichaelZuo 3y agoHow is the system controller initiated? I assume via a simpler process?
- convolvatron 3y agousing something like the 'brownout detector' power controller above. I'm pretty sure I've seen designs (and done this myself), that just puts a little RC on the reset pin. 100ms aughta be enough for anyone! microcontrollers (like those used for board controllers) have a much simpler power and clock structure than big cpus...and many of them are built to just come up on their own.
- MichaelZuo 3y ago> and many of them are built to just come up on their own. I'm not quite sure of this, how do these controllers 'come up on their own' without any reliance on anything else?
- convolvatron 3y agointernal flash for boot, internal power control, internal clock
- MichaelZuo 3y agoIt sounds a bit recursive, wouldn't the internal boot process need the same sort of procedure?
- convolvatron 3y agoI just was looking at https://www.pjrc.com/store/ic_mkl02_t4.html https://www.pjrc.com/store/ic_mkl02_t4.html maybe this will give you some context, its about a small controller booting a larger SOC ARM part
- MichaelZuo 3y agoThanks for the link!