jkeny wrote:
Yea, give it a go - it ain't too difficult.
It allows direct experimenting with these USB hub chips:
- power the USB hub chip & 24MHz clock with 3.3V LiFePo4 battery, directly
- try using synchronous clock sharing between USB hub chip & whatever USB receiver you are using - it could lift the SQ even more? (Nige is trying this & may report soon?)
right ive had this working a couple of days
i built it in such a fashion that i could plug in and out either an asynchronous usb hub or a synchronous one both powered the same so that it was comparable
i had expected it to be better synchronous and initially i was a little confused with the results i found a little grey area
the asynchronous hub seemed to have more detail but more edge to the sound, on more listening i realised the synchronous was more natural/ real sounding albeit a little bit of detail, one thing i dont like to surrender, but my gut feeling is that synchronous is right and maybe down to synchronous needing improved implementation
for the most part results are inconclusive, discussed my findings earlier with jk, the fact is the 48mhz clock and the frequency divider f-f of the synchronous system is running of the xmos built in 1.8v, at least on the asynchronous the hub clk is direct lifepo4. and when you think about it runing clks or even f-fs from the built in xmos 1.8v power supply is just a bad idea
data sheet says oscillator needs to be 1.8v oscillator so 3.3v levels are out, therefore we need a new 1.8v supply? and ideas are welcome
3.3v is our best reference supply so how do we get a 1.8v just as good?
i.e if using an XMOS USB receiver some use a 48MHz clock - divide this in half & use the resulting 24Mhz signal at the USB hub chip
If using an Amanero board which needs a 12Mhz clock for USB duties, divide in half the USB hub chip's 24MHz clock signal & send the resulting 12Mhz signal back to be used as the Amanero board's USB clock.
A clock divider can be done with just a simple 74 flip flop but you do have to take into account the different voltage levels required by the chips clock signal inputs i.e the XMOS requires a 1.8V clock source & the USB2412 can take a clock signal of a minimum 1.4V level - so both should work if using a low voltage flip flop to do voltage level shifting i.e these can accept a 3.3V input clock signal & output a divided down clock at 1.8V
The Amanero is a bit simpler as it doesn't require voltage level shifting, like the XMOS receiver - the ARM chip (which performs the USB duties) uses a 3.3V USB clock input signal as does the USB2412 chip so a standard 74 flip flop should be fine