Confirming understanding of SAI data alignment figures in RT1170 manual

[yfliu]

Dear forum members,

I am learning the SAI module section of the RT1170 reference manual, i.e. the section 58.  However, I found that I can hardly understand the figures 58-2 or 58-3 in section 58.3.4.1 data alignment part.

What I can understand is that only a portion of bits in a 32-bit word is worth to be transferred over SAI. The FBT field is designed to tell the SAI module where to find the valid bits. As SAI supports MSB first or LSB first bit ordering, so the FBT value should be interpreted differently in the two ordering cases. According to FBT field description in 58.5.1.9.3, I can learn that for MSB first case, FBT field specifies the MSB bit of the valid data; for LSB first case, the FBT field specifies the LSB bit of the valid data.

But the figures in 58-2 or 58-3 are a little hard for me. For example, in fig 58-2, 24-bit two FBT values (0 and are illustrated. Does this mean that both FBT values are valid cases for SAI module and it can derive the ending bits using word width field accordingly? If this is true, then developers can choose most convenient form for their apps.

If my above understanding is correct, I would suggest the text around figure 58-2 and 58-3 can be rephrased to highlight that for word length smaller than 32bits, the figure shows two typical alignment schemes (smalles and biggest FBT values).

Regards,

Yanfeng

[kerryzhou · Accepted solution]

Hi @yfliu ,

   About your questions, it is related to the following picture:

This is not difficult to understand.

The FBT is First Bit Shifted, just use 32 bit as base, and different number bits in the different position, you need to consider 3 points:

1. FBT

2. bit numbers: 32,24, 20, 12, 8bit

3. MSB or LSB

The above table give you the details. Associated with the Data FIFO, 

Your question:

1. in fig 58-2, 24-bit two FBT values (0 and are illustrated. Does this mean that both FBT values are valid cases for SAI module and it can derive the ending bits using word width field accordingly? If this is true, then developers can choose most convenient form for their apps.

=>Answer: FBT=0, it means, LSB, 0 bit shift, the 24bit data put from bit 0 in the 32 bit FIFO,  the higher 8 bit is not used.

MSB is the same, if the FBT=10111, it is the 23 bit, it means shift from 23 bit, so you can find the data put from 0-23 bit.

The other situation is totally the same,

Wish it helps you!

BTW, if you work the issues for the company, please use your company email to create the account, and create the question, that will have higher priority.

Best Regards,

kerry