On 23/12/2010 00:03, Philip Pemberton wrote:
> Quick question -- what's the fastest anyone's managed to transfer data
> from a PIC18F85J50 to or from a PC, using USB?
>
> After a heck of a lot of protocol tweaks and the addition of
> double-buffering (which netted me another 20 kbytes/sec), I'm up to a
> shade under 220 kbytes/sec. Is this about right for a chip with the PLL
> on and a core clock of 48MHz?
>
> The data is being sourced from the Parallel Master Port -- data either
> goes from USB to PMP, or the other way.
>
> The source code is online at http://hg.discferret.com/firmware/ for the
> curious. This includes a patch to the Microchip USB Stack which adds a
> 'special' string descriptor that reads the device serial number from the
> end of the bootloader block of the Flash.
>
> Thanks,

It's only USB 1.1 not USB 2.0

USB speeds are fantasy. Nothing gets close to supposed speed.

PIC 18F uses 24MHz USB clock =3D 12MHz peak data rate of USB 1.1
There is CRC overhead on every packet
It's half duplex

220kByte/s is about 1.7Mbps  Not bad really. maybe you might get 4 to=20
6Mbps on USB 1.1 if really clever. Only one data connection, so any data=20
transfer can only use a proportion of the speed with protocol overhead,=20
TX/RX change over and CRC etc.

<< USB 1.1 has a theoretical throughput of 12 Mbps, or 1.5 MB/sec . In=20
practice, the fastest you're likely to see is less than 8 Mbps [but it's=20
half duplex]. This discrepancy arises because the USB controller=20
reserves a portion of the bandwidth for non-data use and because USB=20
uses bit stuffing, a technique that mixes filler bits with the actual=20
data to simplify the system. >>

I've also seen "real world" speeds for USB 1.1 of  5.4Mbits/s average=20
throughput (about 6.1Mbits/s peak)  quoted.



Also the CPU is 12MIPS peak with 48MHz clock. i.e. if you doing 6=20
instructions per byte transferred =3D 2Mbps.
So other than streaming Parallel port mode you are not going to get =20
anywhere near full 1.1 speed.

So 220k bytes /sec doesn't sound too bad.



--=20
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