The ADSP-2191 datasheet has the following statement on page 11 concerning the power-up sequence:
"Power up together the two supplies VDDEXT and VDDINT. If they cannot be powered up together, power up the internal (core)supply first (powering up the core supply first reduces the risk of latchup events."
In the design I'm writing about, both supplies are produced using low-dropout linear regulators with a common +5V input. The intent was to power both supplies simultaneously.
The plot below shows the actual timing of VDDEXT (+3.3V) and VDDINT (+2.5V).
The +3.3V supply appears to follow the curve of +2.5V (at a lower voltage) until it turns on a couple of milliseconds later. This timing (I think) is in agreement with the sequence specified in the datasheet. However, I've found that sometimes the DSP locks up on power up, where nothing, other than a power cycle, can recover it.
Delaying the VDDEXT relative to VDDINT doesn't help; but powering VDDEXT before VDDINT does! Could the datasheet have the sequence backwards? I've searched the errata and didn't find anything related to this.
Thanks in advance.