3 ms·

pretty much all modern boards use DC/DC power stages from 12v, even for 5v and 3.3v stuff.

by throwaway2048 7y ago

baybal2 7y ago
Indeed, found that for myself when got into mobo design. ATX 5V is really unsuitable for USB power, and can play tricks on you. Similarly, 3.3V from PSU is useless at powering 3.3V logic. You have to do power staging on board to guarantee that PCI and i2c stuff turns on after the bridge/soc. Some PSUs don't follow reset timings, or don't do them at all on standby rail. So if you do ATX power by the book, you may end up with improperly inited board, and 3.3V ICs booting up before the CPU. So, yes, most of ATX power supply functionality is already duplicated by most mobos
- mmastrac 7y ago
  > ATX 5V is really unsuitable for USB power, and can play tricks on you. Would you mind elaborating on this?
  - baybal2 7y ago
    Poor transient response. Ringing on some PSUs. Out of spec ripple on most PSUs.
- garaetjjte 7y ago
  >ATX 5V is really unsuitable for USB power, and can play tricks on you. Why? I don't think current motherboard duplicate 5V converter. >you may end up with improperly inited board, and 3.3V ICs booting up before the CPU. Why it cannot just hold reset line for these chips high until CPU started?
  - Gibbon1 7y ago
    A lot of peripheral IC's don't have reset lines. Sometimes this is a big big problem. You can end up with failure to come up maybe 1 in 10,000. Exactly the sort of thing that shows up late in development or worse in the field. Tip: If anyone tells you power supply and reset sequencing is 'easy' ignore them.
  - dezgeg 7y ago
    Holding chips in reset is not the only thing that matters. Complex chips may have power-sequencing restrictions; for example 1.8V power rail might need to be up before 3.3V, otherwise the chip gets damaged.
- chungus_khan 7y ago
  Yeah, unfortunately we've thoroughly decayed past the good old AT days of being able to power logic straight off of the 5V. It helped that in those days there was one IBM making the PCs.

pretty much all modern boards use DC/DC power stages from 12v, even for 5v and 3.3v stuff.

Indeed, found that for myself when got into mobo design. ATX 5V is really unsuitable for USB power, and can play tricks on you. Similarly, 3.3V from PSU is useless at powering 3.3V logic. You have to do power staging on board to guarantee that PCI and i2c stuff turns on after the bridge/soc. Some PSUs don't follow reset timings, or don't do them at all on standby rail. So if you do ATX power by the book, you may end up with improperly inited board, and 3.3V ICs booting up before the CPU. So, yes, most of ATX power supply functionality is already duplicated by most mobos

mmastrac 7y ago
> ATX 5V is really unsuitable for USB power, and can play tricks on you. Would you mind elaborating on this?
- baybal2 7y ago
  Poor transient response. Ringing on some PSUs. Out of spec ripple on most PSUs.
garaetjjte 7y ago
>ATX 5V is really unsuitable for USB power, and can play tricks on you. Why? I don't think current motherboard duplicate 5V converter. >you may end up with improperly inited board, and 3.3V ICs booting up before the CPU. Why it cannot just hold reset line for these chips high until CPU started?
- Gibbon1 7y ago
  A lot of peripheral IC's don't have reset lines. Sometimes this is a big big problem. You can end up with failure to come up maybe 1 in 10,000. Exactly the sort of thing that shows up late in development or worse in the field. Tip: If anyone tells you power supply and reset sequencing is 'easy' ignore them.
- dezgeg 7y ago
  Holding chips in reset is not the only thing that matters. Complex chips may have power-sequencing restrictions; for example 1.8V power rail might need to be up before 3.3V, otherwise the chip gets damaged.
chungus_khan 7y ago
Yeah, unfortunately we've thoroughly decayed past the good old AT days of being able to power logic straight off of the 5V. It helped that in those days there was one IBM making the PCs.