Hello, In my future design I could win a lot of pins if I could drive a bus at 160MHz. Because of bank restrictions and because this bus is connected to a CPLD, I will have to use LVTTL. Has anybody tried driving a bus in LVTTL at 160MHz? I would prefer to use LVDS but the CPLD doesn't allow that. Lattice has LVDS CPLDs but only the large CPLDs support LVDS inputs. I am afraid that this bus will have a lot of EMI/EMC problems. What do you think of it, should series termination be adequate to limit the EMI/EMC problems? best regards, Dolphin
Driving a 30 bit wide LVTTL bus at 160MHz
Started by ●September 27, 2006
Reply by ●September 27, 20062006-09-27
Simulate it. Hyperlynx is your friend! HTH, Syms. "Dolphin" <Karel.Deprez@gemidis.be> wrote in message news:1159368842.453549.244660@b28g2000cwb.googlegroups.com...> Hello, > > In my future design I could win a lot of pins if I could drive a bus at > 160MHz. Because of bank restrictions and because this bus is connected > to a CPLD, I will have to use LVTTL. > Has anybody tried driving a bus in LVTTL at 160MHz? > > I would prefer to use LVDS but the CPLD doesn't allow that. Lattice has > LVDS CPLDs but only the large CPLDs support LVDS inputs. > > I am afraid that this bus will have a lot of EMI/EMC problems. What do > you think of it, should series termination be adequate to limit the > EMI/EMC problems? > > best regards, > Dolphin >
Reply by ●September 27, 20062006-09-27
Dolphin wrote:> Hello, > > In my future design I could win a lot of pins if I could drive a bus at > 160MHz. Because of bank restrictions and because this bus is connected > to a CPLD, I will have to use LVTTL. > Has anybody tried driving a bus in LVTTL at 160MHz? > > I would prefer to use LVDS but the CPLD doesn't allow that. Lattice has > LVDS CPLDs but only the large CPLDs support LVDS inputs. > > I am afraid that this bus will have a lot of EMI/EMC problems. What do > you think of it, should series termination be adequate to limit the > EMI/EMC problems? > > best regards, > Dolphin >It can be done with extreme care. Keep the lines short, you may need to terminate them, and by all means do a SI simulation on it so that you know it will work rather than blindly applying "fixes".
Reply by ●September 27, 20062006-09-27
Also be careful of Simulaneously Switching Outputs guidelines. As Ray says it can be done but if you use a crappy package like PQ208 or TQ144 you may have big problems with ground bounce that causes the interface to fail. Most of the BGA packages do much better at this fast type of driving from our experience. Some CPLDs like the bigger Coolrunner-IIs can also support single ended standards like SSTL, HSTL that are designed for high speed and may be a an alternate solution for you. These standards have the advantage of small signal swings too which is better for ground bounce and EMC. John Adair Enterpoint Ltd. Ray Andraka wrote:> Dolphin wrote: > > Hello, > > > > In my future design I could win a lot of pins if I could drive a bus at > > 160MHz. Because of bank restrictions and because this bus is connected > > to a CPLD, I will have to use LVTTL. > > Has anybody tried driving a bus in LVTTL at 160MHz? > > > > I would prefer to use LVDS but the CPLD doesn't allow that. Lattice has > > LVDS CPLDs but only the large CPLDs support LVDS inputs. > > > > I am afraid that this bus will have a lot of EMI/EMC problems. What do > > you think of it, should series termination be adequate to limit the > > EMI/EMC problems? > > > > best regards, > > Dolphin > > > It can be done with extreme care. Keep the lines short, you may need to > terminate them, and by all means do a SI simulation on it so that you > know it will work rather than blindly applying "fixes".
Reply by ●September 28, 20062006-09-28
Hi, maybe this is a stupid question, but is there any chance to absorb the cpld logic into the FPGA? this will move your interface inside and make your life easier. Aurash Dolphin wrote:>Hello, > >In my future design I could win a lot of pins if I could drive a bus at >160MHz. Because of bank restrictions and because this bus is connected >to a CPLD, I will have to use LVTTL. >Has anybody tried driving a bus in LVTTL at 160MHz? > >I would prefer to use LVDS but the CPLD doesn't allow that. Lattice has >LVDS CPLDs but only the large CPLDs support LVDS inputs. > >I am afraid that this bus will have a lot of EMI/EMC problems. What do >you think of it, should series termination be adequate to limit the >EMI/EMC problems? > >best regards, >Dolphin > > >-- __ / /\/\ Aurelian Lazarut \ \ / System Verification Engineer / / \ Xilinx Ireland \_\/\/ phone: 353 01 4032639 fax: 353 01 4640324
Reply by ●September 28, 20062006-09-28
Dolphin wrote:> Hello, > > In my future design I could win a lot of pins if I could drive a bus at > 160MHz. Because of bank restrictions and because this bus is connected > to a CPLD, I will have to use LVTTL. > Has anybody tried driving a bus in LVTTL at 160MHz? > > I would prefer to use LVDS but the CPLD doesn't allow that. Lattice has > LVDS CPLDs but only the large CPLDs support LVDS inputs. > > I am afraid that this bus will have a lot of EMI/EMC problems. What do > you think of it, should series termination be adequate to limit the > EMI/EMC problems?You mentioned speed, but forgot distance ? How far does this have to go - sounds like the CPLD is a (remote?) slave ? You can get small LVDS-LVTTL transcievers, plus you can also use dual data lines, to halve the clock rate. ( see winbond for dual-data SPI memory, they spec to 150MHz ) -jg
Reply by ●October 3, 20062006-10-03
Add a series-resistor in the lines and/or use the current-limiting feature of the I/O-ports (if available). (too reduce the slopes of the signals) Kind regards, Stef? "Dolphin" <Karel.Deprez@gemidis.be> schreef in bericht news:1159368842.453549.244660@b28g2000cwb.googlegroups.com...> Hello, > > In my future design I could win a lot of pins if I could drive a bus at > 160MHz. Because of bank restrictions and because this bus is connected > to a CPLD, I will have to use LVTTL. > Has anybody tried driving a bus in LVTTL at 160MHz? > > I would prefer to use LVDS but the CPLD doesn't allow that. Lattice has > LVDS CPLDs but only the large CPLDs support LVDS inputs. > > I am afraid that this bus will have a lot of EMI/EMC problems. What do > you think of it, should series termination be adequate to limit the > EMI/EMC problems? > > best regards, > Dolphin >