Question
We are using an AD9520-3 Clock generator in one of our designs, but
unfortunately we are experiencing some problemen with respect to the loading of
the eeprom settings, stored inside the AD9520's eeprom memory.
We have pulled the eeprom enable input high so it should load from the inside
eeprom.
If loaded correctly everything works fine, but sometimes, one in 10 startups or
so, the eeprom settings aren't loaded and the AD9520 remains configured with
it's default settings. This situation is offcource highly unwanted, because the
chip doesn't function as expected in such a situation.
Strangely enough I'm also able to replicate the problem by invoking a lot of
reset pulses on the reset pin, while keeping the chip powered on, then also
once in a while the AD9520 doesn't load its eeprom setting. It seems like some
form of faulty eeprom loading / reset recognition accurs. What I mean with a
lot of reset pulses: we've put a reset button on the PCB, by pressing it you
ground the reset input of the AD9520, if you press it really often in a short
period of time, we're also able to reproduce the problem, the "fail state" then
occurs once in a while. Maybe it has something to do with contact bounce of the
reset signal?
I would really appreciate any help, I roughly followed the design of the AD9520
evaluation board and studied the datasheet, but I'm not able to see any
possible cause of the problem there. There is no mentioning of special reset
conditions or circuitry that should be required.
Answer
The threshold voltage at which the AD9520’s power on reset triggers is a little
on the low side if the power supply ramp is slow.
We recommend leaving the RESETb line high during power-up until after the power
supplies are stable, and then use a >=20ns low pulse on RESETb.
We’ve had a few other customers with this issue, and this has solved the
problem.
Reset button on the PCB: Don’t do that. You can hang the RESET of the part or
hang the EEPROM I2C controller.
On other comment: the input clock must be present when the EEPROM is loading,
as the VCO calibration depends on an input clock.