national semi LVDS( DS90CR217&DS90CR21A ) problem

[yangvic]

I use national semi LVDS( DS90CR217&DS90CR21A ) to connect two GALILEO GT48520 GE MAC.I use Smartbits test it. BUT if packet size is below 300Byte and 100% traffic.Between this two GT48520 transfer will lose.This LVDS track's length is about 30CM.I have checked everything that i can.BUT i have no idea.Somebody can help me?TKS

 

[MZanders]

I don't understand your explanation completely
but LVDS TX/RX needs a minimum datarate to operate correctly. Is made clear in the datasheets though...

 

[yangvic]

how to tie unused LVDS pin?our LVDS unused input and output pins all do not connect.

 

[dumbfrog]

the unused input pins for cmos/ttl should tie it to ground and the lvds input pins should let it float (there should be fail-safe).

there operating frequency for this device is 20-85MHz, the pll won't work, difficult to lock, below 10MHz or over 100MHz.

check your current consumption of the chip
check the output of the lvds driver, both clock and data
make sure you have 100Ohm termination on the receiver side
what is your loading? don't exceed 5p....that's difficult.
pwdn' should be high

 

[yangvic]

Could this LVDS transmit line reach 100cm in FR4?Our LVDS line is about 70CM.

 

[yangvic]

sorry!our clock is 83MHz.TKS!

 

[dumbfrog]

100cm=1meter the trace capacitance is too big, maybe over 30p
typical driver is 4mA and personally, I don't think it will work.

i have tried the 387/388 chip with a 3meters and a 5meters cable, but with a buffer (a lvds to lvds) to boost up the driver.

 

[chinara]

It is strange for your description. I also meet same problem before, but this is an important technique of our company. Please send me a message if you do not solve the problem until now.