AnsweredAssumed Answered

ADV7611 - start up, oscillator

Question asked by LukaszNowak on Aug 31, 2012
Latest reply on Sep 4, 2012 by mattp

Hi again,

 

I'm starting this thread because i have some problem with ADV7611 startup. Since the last post I've made some progress, so staring from the beggining the story goes:

- after the first power up, I2C communication could not be established (0x98 neither 0x9A adresses)

- I've measured my power supply independently before, disconnected from the circuit - the levels and startup sequence are fine (1V8 comes up 2ms after 3V3 is established)

- 2 seconds after power up, the ADV_RESET goes HIGH and imedietaly after strange behaviour comes: on all ADV7611 1V8 terminals a higher voltage of ~3V appears (which makes my 1V8 powers supply rise to ~2,3V).

- I've suppressed the level on 1V8 to ~1,9V by placing additional load resistor, hoped that the supply makas it due to low load. But this happens even when 1V8 rail is disconnected from the chip (with 3V3 connected only)!

- I've found that no oscillaions appears on the xtal

- had nothing esle in mind, I've put external 28,6363MHz signal to the XTAL terminal. Due to temporarly having only some **** generator with no offset funcion, I've put the resistor biasing cicuit to the XTAL terminal to get ~1V bias.

- I run the software and... the comunication worked out! BUT the reason was not the external source olny placing the resistor 500ohms from XTALN to GND. With removed xtal chip as well as external clock source, and only having this 500 resistor, the higher voltage on 1V8 is gone and I2C responded. There are no ocillations, but for the I2C data read i guess it is not needed.

 

So the reason sems to be something with the oscillator circuit. I will give more details about the used xtal and load caps when I am back at work on Monday.

I've checked the pin assigment and connections on layput, and they are the same as on AD eval board. The GND pad is also soldered properly.

This behaviour appears so far on two boards.

Any clue by now what could be the cause? I will try maybe with difeerent xtal and load caps o Monday.

 

Have a nice weekend!

Lukasz

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