ADAU1446 CLKOUT modulating

Hi. I've built a prototype using the ADAU1446 using the CLKOUT as fs*256 to clock a CODEC. It's working and I'm not conscious of any problems with the output signals that might be caused by the master clock. However, CLKOUT appears to be frequency modulating at about 24kHz (fs=48kHz), the oscillator itself is stable. As best as I can tell fs is also stable. Any ideas? Is this something I'm likely to see using the fs*256 option? Would I be better using the buffered oscillator? tia Gordon

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  • Hi Brett. Disconnected everything from the CODEC, no change. If I reduce the schematic to it's bare minimum, the 'jitter' reduces. I can't get a decent scope shot, sorry. Project file attached. There isn't really much to the circuit once the codec's disconnected. It's running at the default 48k with component values for PLL as per ADAU1446 spec sheet. CLKOUT as I've said is running at 256*fs, CLKMODE0=LOW, CLKMODE1=HI. RSVD and RST tied low. SPDIFO is driving one of these http://tinyurl.com/y8uj6rz. SDATA_IN0 is the only input. MP10 has a 10k pullup. All other MP pins are open as they go out to a header which isn't plugged in. CLATCH is tied to ground. It's a two layer board and all chip power pins are decoupled as close as I could manage to the chip. No uP, just an EEPROM which is working/self booting OK. The crystal is the same one used on the ADAU1701 evaluation board. Remember, it only 'jitters' once it's been compiled.

    Won't expect to hear from you before Monday.

    Have a good weekend.

    Gordon

    1446-Modulation.dspproj.zip
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  • Hi Brett. Disconnected everything from the CODEC, no change. If I reduce the schematic to it's bare minimum, the 'jitter' reduces. I can't get a decent scope shot, sorry. Project file attached. There isn't really much to the circuit once the codec's disconnected. It's running at the default 48k with component values for PLL as per ADAU1446 spec sheet. CLKOUT as I've said is running at 256*fs, CLKMODE0=LOW, CLKMODE1=HI. RSVD and RST tied low. SPDIFO is driving one of these http://tinyurl.com/y8uj6rz. SDATA_IN0 is the only input. MP10 has a 10k pullup. All other MP pins are open as they go out to a header which isn't plugged in. CLATCH is tied to ground. It's a two layer board and all chip power pins are decoupled as close as I could manage to the chip. No uP, just an EEPROM which is working/self booting OK. The crystal is the same one used on the ADAU1701 evaluation board. Remember, it only 'jitters' once it's been compiled.

    Won't expect to hear from you before Monday.

    Have a good weekend.

    Gordon

    1446-Modulation.dspproj.zip
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