Xilinx Spartan6 IO 电平读书笔记

XilinxSpartan6IO电平读书笔记FPGA的IO支持多种电平标准,但是其中用几点的概念比较模糊,在此特意记下:最近在用xilinx的spartan6与ARM进行通信，但是FPGA的逻辑电平是3.3V的LVTTL标准,而ARM输出的是1.8V的电平标准，两者IO电平的不匹配，出现了一些问题如下：FPGA在VCCO=3.3V是，是否可以设置FPGA的IO电平标准为1.8V？在翻阅了《Spartan-6FPGASelectIOResources》后，得到了以下信息：Spartan-6的供电来源于3个引脚：VCCINT、VCCO、VCCAUX。VCCINTisthemainpowersupplyfortheinternalFPGAlogic.VCCINTalsopowerssomeoftheavailableinputdrivers.VCCINT主要为FPGA的内部逻辑单元供电，同时也会给一些输入供电。TheVCCOsupplies,oneforeachoftheI/Obanks,powertheoutputdriversandsomeoftheinputdrivers.ThevoltageontheVCCOpinsdeterminesthevoltageswingoftheoutputsignal.Manyofthelow-voltageI/OstandardssupportedbySpartan-6devicesrequireadifferentoutputdrivevoltage(VCCO).Asaresult,eachdeviceoftensupportsmultipleoutputdrivesourcevoltages.OutputbufferswithinagivenVCCObankmustsharethesameoutputdrivesourcevoltage.ThefollowingI/OstandardsinputbuffersalsousetheVCCOvoltagesupply:?LVCMOS25(whenVCCAUX=3.3V)?LVCMOS18_JEDEC?LVCMOS15_JEDEC?LVCMOS12_JEDEC?PCI?MOBILE_DDRSpartan-6FPGAsallowmultipleI/Ostandardstobecombinedinthesamedevice.AlthoughtheoutputsarealwayspoweredbyVCCO,multiplestandardsareavailableunderoneofthefivepossibleVCCOvalues.Inaddition,inputsoftendonotneedtomatchthevoltageappliedtoVCCO.FurtherflexibilityisachievedwithmultipleVCCOlevelsinasingledevice.EachbankofI/OshasindependentVCCOandVREFrails.ThisallowseachbanktobepoweredatVCCOandVREFlevelsindependentofhowtheotherbanksareset.VCCOprovidespowerprimarilytotheI/Ooutputbuffers,andVREFsuppliesareferencevoltageforHSTLandSSTLinputs.TheVCCOpinsarededicatedpowerpinsandmustbepoweredatalltimeswithavoltagerailfromthePCB.However,theVREFpinsaredual-purposepins;theycanbeusedasregularI/OpinsorVREF-supplypins.WhenabankusesVREF-poweredinputs(asanexample,fortheSSTLorHSTLstandards),thedesignmustusetheVREFpinstosupplytheFPGA’sinternalVREFrailwiththereferencevoltage.IftheSSTLorHSTLinputsarenotusedinabank,theVREFpinsinthatbankcanbeusedasregularI/Opins.Table1-5liststheVCCOandVREFrequirements.VCCO在FPGA中，每个Bank都有自己独立的VCCO，每个Bank的VCCO可以供不同的电源，以实现不同的Bank实现不同的IO标准，在同一个Bank中，VCCO只能是一个标准，通过上面的解释中，可以看到，IO的输出都是由VCCO供电的，也就是说每个Bank的IO输出电压决定于VCCO，同时部分标准的输入也