Re: [usb] Clarification on USB

["Vikas T Rao " <vikasraot@myw.ltindia.com>]

** Proprietary **

hi ravi,

if u r operating at 12Mbps, it's optimum to use 48MHz. u can use any frequency in the multiples of 12 i.e., u can use 12/24/36/48/60/72 and so on. but 48 is chosen probably because,

1. even multiple of frequencies(24/48..) will give 50% duty cycle when used for clock extraction. 50% duty cycle will have optimum power consumption.
2. 48MHz gives adequate sampling of the incoming data stream. there is no chance of missing the data stream even with 3 samples of 48MHz. while 24MHz can be used for only 2 samples.
3. if u have use more frequency than 48MHz(say 60/72), then PCB routing problems occur. higher clocks generally create EMI. this adversly affects the other signals. to minimize these EMI, PCB routing has to be very stringent than routed for 48MHz.

if u consider all the above options, 24MHz is a great competitor for 48MHz. even i wanted to know clearly why 24MHz can't be used for 12Mbps.

regarding ur question for low/high speed devices, the same reasoning as above applies for 1.5Mbps but not for 480Mbps. for 480, generally PLLs are used. so, 30/60MHz crystals are used to multiply to 480MHz. only serial operations(NRZI, stuffing, CRC) are done at 480MHz but rest of the parallel operations(other logic than identified above like data loading into memory etc) will be done at 30/60MHz only.

now a PLL can be used as above for 1.5/12Mbps to achieve 6/48MHz. but u have to consider trade-off between using a PLL(additional logic for multiplication etc) and 48MHz(PCB routing etc).

spec doesn't say to use any particular clock frequency. chosing it is upto u for doing all USB operations reliably and optimally according to ur requirements.

by the way, where r u working now?

...vikas.


>>> ravikumar_vlsi@yahoo.co.in 01/27/03 06:51PM >>>
Hi Folks,
I have a clarification in USB.
Would some pls tell me how they arrive at 48 Mhz for
USB Clock.
I vaguely understand that itz due to considering some
.25% tolerence .
Pls explain me with some detail for two scenarios

1.If am operating for only low speed Devices, what is
the frequency of 
USB Clock required

2.1.If am operating for only High speed Devices, what
is the frequency of 
USB Clock required

Regards
Ravikumar

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