ALTLVDS_RX few questions.

Question

http://www.alteraforum.com/forum/attachment.php?attachmentid=10549&stc=1

hey guys, I have finally gathered the will to use an fpga. the plan is to use an octal adc to receive serialized data, which will later on be processed inside an cyclone II eval board. the lvds serial interface from the adc is dual data rate with two lvds pairs, one pair providing the first 7-bits and the second pair providing the next 7-bits of a 14 bit word. I want to use the altlvds_rx to parallelize this data. I would like to know:

a) if the altlvds_rx megafunction can resolve DDR data or is it intended for one bit per clock cycle.

b) can i use a single altlvds_rx module to capture data from a 2 pair lvds system described above.

c) i have tried several times but cannot get my head around deserialization factor. how would i calculate the deserialization factor to parallelize two 7-bits serial streams to a 14 bit parallel word.

I am attaching the block diagram of the adc, it can operate in both 1 or 2 lvds pair per channel mode and has a bit clock and a word clock output.

Thanks very much.

altera_forum · Answer

You would have 2 inputs each with a 7-bit deserialization factor. This would give you two 7 bit data buses of which one is the upper 7 bits and one is the lower 7 bits. These are then concatenated together to make the 14bit wide data bus.

Assuming you have a frame clock, then you don't use DDR as such, but use a PLL to generate the bit clock from the frame clock at a high enough frequency to clock you data stream. If the data is DDR, you may have to use the ALTLVDS module in external PLL mode and instantiate a PLL instance for your required frequencies.

altera_forum · Answer

thanks TCWORLD, can i be really adventurous and add 16 LVDS pairs (from the 8 ADC channels) into a single altlvds_rx receiver block, which in turn is fed be an external pll, or am i speaking plain stupidity. opinions will be much appreciated. thanks.

altera_forum · Answer

You should be able to, but you need to check the FPGA datasheet. There are rules about which SERDES blocks can be fed by which PLLs. If your 16 LVDS inputs cannot be fed by the same PLL, then you will have to break the 16 down into smaller blocks.

I've use 8 LVDS pairs in a block before. I could have used 16 on the same PLL in one of the Stratix IV's. I don't know about the Cyclone II's .

altera_forum · Answer

thanks TCWORLD,  cyclone II has so called IOE's but no SERDES block from the looks of it. the altlvds_rx is implemented using LE's within the device, don't know how good or bad this is? if i find anything about pll fan out in cyclone II i shall report back. thanks.

altera_forum · Answer

Ah right, in which case you can make the ALTLVDS block as many channels as you have logic elements for. What you will find though is heavy restrictions on the Fmax as deserialization is implemented in logic. Only time(quest) will tell if it will be fast enough for your applicaiton.

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ALTLVDS_RX few questions.

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