The following power-up sequence is recommended for i.MX25.
1. Assert power on reset (POR).
2. Turn on digital logic domain and I/O power supplies VDDn and NVCCx.
3. Turn on all other analog power supplies
4. Negate the POR signal
So our board designed as following.
(0) POR=L (1) LDO_1.2V on (2) LDO_3.3V on (3) LDO_1.8V on (4) POR release
Most boards(90%) started up with no problem and stable but rest of boards(10%) looked unstable.
Please see attached waveforms.
LDO_1.2V (ch1: yellow) rise first then LDO_3.3V (ch2: cyan) rise next but oscillation waveform observed.
(Additionally saying, this oscillation seems suppressed if add delay to the enable pin of LDO_3.3V.)
Why this type of oscillation happened in few specific boards?
How can I fix it? Is there any hardware design guide documents?
Can anyone help me?