Help with clock division question.

K8JLF · 2004-12-05T15:33:15+00:00

I've been messing around with a communications board lately which can have T1or E1 interfaces. I'd like to change one into the other. Naturally I'll need tochange the framing IC's (and a few other things not related to timing), but Ithink for it to run correctly, I need to also change the clock driving them.Stuff I know:The T1 framer uses a 1.544mhz master clockThe E1 framer uses a 2.048mhz master clockThe main clock for the whole board is 50.000mhz.There is only one clock crystal on the board, so everything must be derivedfrom this crystal I would think. There are no PLL chips on the board that I can immediately identifyThere ARE flip-flops on the board, but I don't see how a flip flop would beused to generate either of these clock frequencies from a clock of exactly50.000mhz, since 50.000 does not divide down evenly into either 1.544 or 2.048There ARE quite a few FPGA's on the board, and the identity of the board(whether it thinks it's T1 or E1) is determined by the contents of a EEPROM.My theory right now is that something in firmware looks at what the boardidentifies itself as, and decides how to set something in an FPGA that producesthe clocks. Does this seem feasible?Also, what other methods could it be using to get 1.544mhz and 2.048mhz from50.000mhz?The framing IC's in question are Dallas/Maxim DS2152L (T1) and DS2154L (E1). Ihave datasheets if anybody wants to have a look.-C

Dec 05, 2004 21 Replies

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Terry Given 21 years ago

buried inside them FPGAs no doubt - state machines can eat jobs like this.

Good luck!

Cheers Terry

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Nico Coesel 21 years ago

Why not use the Zarlink MT9074 framer/transceiver? This one does E1 and T1 in one chip AND runs from a 20MHz clock source (it has an internal PLL). This saves board space and 20MHz is easier to create from 50Mhz. Create 40MHz by skipping one cycle every eight cycles at

50MHz and divide by 2 to create a symetric 20MHz clock. I've used the MT9075 (the E1 only version) for several projects and found it quite easy to use and straightforward to program.

Reply to nico@nctdevpuntnl (punt=.) Bedrijven en winkels vindt U op www.adresboekje.nl

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