TDM Data Calculating

Hello Elec23,

Please do not take my writing in any bad way since I cannot use tone-of-voice in a forum post!

You are not setting this up correctly and not thinking through this problem fully. Let me see if I can help...

First, I need to answer your question about the DAC. The DACs have no buffer with the exception of the shift register to shift in the data from the serial port. Once the data is completed at the end of the sample period, the data is shifted into the DAC and it outputs the level based on the number it received. So if you send it zeros it will put out zeros. If it suddenly changes then the output will suddenly change and the oversampling filters will ring because of that. It will not be good. 

The problem needs to be split up into two different sections. The input and the output. I know you are aware of this. 

since the data is coming in big chunks every 4ms you need to have a buffer capable of capturing 4ms of samples. At 48kHz that is one sample every 20.8us. So 4ms is exactly 192 samples. this is for only one channel.  For all eight channels that is 1,536 samples. You may want to have two buffers you swap, one for incoming data and one for outgoing data but that all depends on how dependable the timing all is. So this is the input side. Fill up the buffers. 

Then for the output side you have to setup a timer/counter or however your sampling rate is setup to interrupt the processor every sample period. 

I cannot get too much more specific since I do not know some important details about your system. But you then have to load in the serial port output shift registers with eight channels of data every sample period. Then if you are bit-banging the TDM interface you will have to setup another interrupt on the bitclock line to tell you when you need to output the next bit. You do this until all 256 bits are shifted out. 

Based on this number I am assuming you are setting up the TDM to be eight channels, TDM-8 and with 32 bits in each channel slot. That would be 256 bits shifted out every sample period. 

Once the frame ends (frame means the sample period) then you load up the next 8 samples and away you go!

I have this old past on how to capture these signals on a scope. I really need to write another one of these for TDM. This one is just for I2S which is a two channel standard but the basic concepts are the same. 

(+) How to Take Meaningful Screenshots of I2S Audio Signals - Q&A - Audio - EngineerZone (analog.com)

The TDM screenshot you showed does not look correct but it is not detailed enough to really see much. 

Dave T