STM32F4 PLL I2S continous clock generation

[Hans hubert]

Posted on March 12, 2017 at 00:13

Hello,

i have a question about MCLK and LRCK generation on an STM32F4 system.

I want to connect an Audio ADC to an SPDIF codec which need an continous

Mclk and Lrclock. I have the option to use an external 24.5760 Meg crystal and do

the LRClock phaseadjustment via ttl logic /divider to keep all in sync.

What i need is the I2S_MCLK and I2S_WS (LRCLK) clocks to remain active even when i am

not sending bytes or receive some.

I have not found any information on how to route the PLL clock to an I/O pin.

Is this possible and do i have jitter improvements or should i keep the crystal ?

Big thanks !

[waclawek.jan]

Posted on March 15, 2017 at 20:46

I want to connect an Audio ADC to an SPDIF codec

What exactly is the role of STM32 in this? Can you draw a block schematics, indicating the interconnections and the expected source of clocks and data?

JW

[Hans hubert]

Posted on March 16, 2017 at 08:38

Hello Jan, thanks for reply. I want to use the STM32 for USB Audio stuff.

Please find a block diagram attached. Goal is to replace the clocking circuit (mclk,lrck,slck)by the STM32 PLLI2S. Because

its much easier to adjust the needed samplerates on the STM32 and i save some control ports.

As far as i know continous clocking is only possible by sending zero bytes to the I2S.STM32 read of the adc datas should

be possible in this configuration.

Thank you

[waclawek.jan]

Posted on March 16, 2017 at 11:02

Hans,

this arrangement - I2S in STM32 acts purely as a slave - of course has no chance not to work... (as long as the external SCLK is *permanently* 16 or 32 ticks per LRCK flip - inserting or removing a single SCLK needs you'll need to restart the I2S in STM32 - in this the I2S in STM32 does not implement the protocol properly, ie. it does not restart its internal bitcounter upon every LRCK=WS edge, see errata).

You don't need to connect MCLK to STM32 at all - MCLK is needed only in the ADC and DAC for the digital reconstruction filters; I2S as a digital interface needs only SCLK and LRCK.

--

You can use STM32 also as a master and clock source if you set I2S as a master. I honestly don't know if clock generation in master receiver requirest continuous writing to the data register - probably not, and it's easy to try, but even if it would, a simple DMA in circular mode would take care of it with no need for software intervention.

In the 'F407, there is a dedicated I2S PLL. You can output its clock directly onto some of the MCO pins, see the RCC chapter (the block schematic at its beginning is very useful in these questions). However, you don't need that - the PLL feeds into the I2S which can output a properly divided MCLK onto one of the pins (and then both SCLK and LRCK of course).

I am not sure whether the I2S PLL in STM32 can be fine-tuned on the fly.

JW

[Hans hubert]

Posted on March 16, 2017 at 11:54

Big thanks Jan, to point me to the right direction. I have just read the RCC section of the STM32F429 i can output a PLLI2S clock on pin PC9.That

helps me a lot and is a nice external board clocking feature. I will try it with some of my discoverys. I hope i can change the pll clock on the fly. Oh i see... Mclk is output only

Thanks again Jan.