Time domain/Delay line UARTs - high speeds

Jim Granville wrote: [Old topic Issues on Shift Register in a Clockless UART ] > That design can be done with a delay line ( which needs baud-precision - not really a common building block...), plus it's not clear how it would manage sync in packed streaming data... >

( I've changed the topic, as that thread was looking like someone's homework.)

I think there is scope to study more what can, and can't be done with delay lines. FPGAs cannot clock above 1GHz, but they CAN resolve time to ~140ps regions, in some cases ( see Peter A's posts ).

Taking the example above, delay line bit sampling is easy enough, but the byte sync is rather harder. I think the only solution is a longer delay line, and wider stop bits - so the data is framed as 8(+) Stop bits, 1 start Bit, 8 data bits... (8 bits is illustrative only)

A capture is triggered only when the delay line taps show

8 stop bits, and one start - otherwise the data 'leaving' the delay line can trigger false captures.

Txmit would be via a register chain/MUX tree, that loads/holds until flipped from 'wait' to 'delaylinego', then data would stream out. Pacing would be done from a slower clock, but the time domain precision of this would be delay element sized (~140ps) This assumes, of course, that access to this detail is possible in the FPGA hardware :)

All up, data rates in the GigaBaud region would appear do-able.

There are benefits to thinking more in time-domain, rather than simple MHz - once the ideas are there, the tools and fabric can follow.

-jg

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Jim Granville
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Let me describe today's and tomorrow's capabilities. Inside the FPGS, the general-purpose routing structure makes it difficult to clock GP logic much faster than 500 MHz, but dedicated functions can be pushed faster, perhaps to 800 MHz. On-chip clock delay can be completely eliminated, and global clock skew can (hopefully) be kept below 150 ps. Clock phase shifting can have From: Jim Granville

Reply to
Peter Alfke

I meant to say that Virtex-IIProX has 10+gigabit/sec dedicated transceivers. There is and will be no SpartanProX. Sorry for the typo :-( Peter Alfke

Reply to
Peter Alfke

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