Can the GLOBAL_EN input on CM4 be used as a "power-good" signal to CM4? As in, does it make sense to keep GLOBAL_EN pulled low externally (for example, by a voltage monitor IC) after power-on to delay the SoC boot until the +5V supply stabilizes?
Background:
I'm investigating an issue with a custom carrier board, where the CM4 will sometimes not boot properly after power is applied (I can see from the UART messages that the bootloader loads the kernel image but then hangs indefinitely). So far my best guess is that the cause is the power on sequence. The +5V will sometimes not come up as cleanly as required by the CM4 datasheet (rise monotonically above 4.75 V and stay above 4.75 V for the duration of the operation). The best I can replicate the symptoms is with an artificial lab supply sequence that oscillates the supply around 4 V for a few times after power-on before settling at 5 V.
It's not feasible to fix the power-on behavior of the +5V power supply on the carrier board. One solution I see would be to put a MOSFET between the +5V supply and the CM4 that only opens when the supply is stable. But I'm wondering if a simpler solution wouldn't be to instead just keep GLOBAL_EN pulled low until the supply is stable. It makes sense if this pin (as I understand it) is an enable input for the DC-DC converter on the CM4 that is then powering the SoC.
However the datasheet isn't clear about the function of this pin in such a context and most discussions I find on-line are about resetting the CM4 or using GLOBAL_EN for power saving purpose.
I was wondering if anyone has any thoughts on this before I get to prototyping.
Thanks.
Background:
I'm investigating an issue with a custom carrier board, where the CM4 will sometimes not boot properly after power is applied (I can see from the UART messages that the bootloader loads the kernel image but then hangs indefinitely). So far my best guess is that the cause is the power on sequence. The +5V will sometimes not come up as cleanly as required by the CM4 datasheet (rise monotonically above 4.75 V and stay above 4.75 V for the duration of the operation). The best I can replicate the symptoms is with an artificial lab supply sequence that oscillates the supply around 4 V for a few times after power-on before settling at 5 V.
It's not feasible to fix the power-on behavior of the +5V power supply on the carrier board. One solution I see would be to put a MOSFET between the +5V supply and the CM4 that only opens when the supply is stable. But I'm wondering if a simpler solution wouldn't be to instead just keep GLOBAL_EN pulled low until the supply is stable. It makes sense if this pin (as I understand it) is an enable input for the DC-DC converter on the CM4 that is then powering the SoC.
However the datasheet isn't clear about the function of this pin in such a context and most discussions I find on-line are about resetting the CM4 or using GLOBAL_EN for power saving purpose.
I was wondering if anyone has any thoughts on this before I get to prototyping.
Thanks.
Statistics: Posted by avian — Tue Dec 17, 2024 5:43 pm — Replies 0 — Views 21