AM335x VDDS_DDR decoupling capacitors

You can divide the decoupling caps into 2 types:

a) small ones (size 0201 etc) with about 100nf, for local buffering

b) large ones (size 603 etc) with about 10uf, for global buffering.

Regarding the large ones, the only interesting parameter is the sum of the capacitance. There is no difference between 5 x 10uF and 1 x 47uF.

Regarding the small ones, the only interesting parameter is the low impedance. You can do this with many small capacitors, you can also do this

with ground/vcc planes in your layout. You may even use NO small capacitors if you use ground/vcc planes with impedances of approx. 1 Ohm.

So, it's all a question about how is your layout designed.

regards
Wolfgang

Thanks for the reply.

Ok but I just wanted to know why it is different and why there is no issue in beaglebone doing other way. But I agree I will follow the datasheet for my first PCB to avoid issues.

You need a distance between ground and vcc planes of 50um FR4/prepeg.

Beaglebone Black, by the way, has 14x100nF + 2x10uF decoupling on the DDR chip, not 9 as you say. Starter Kit has 12x100nF + 4x10uF and not 21. Generally speaking you will find it's almost impossible to place all the recommended capacitors close enough to power supply pins under the DDR, but you should try to place as many as possible.

Ok but in the stackup I dont see this kind of value.

Here is the stackup of the beaglebone black: