5v TTL to/from LVTTL

[Altera_Forum]

I need to work with ancient 5v TTL digital components built on 74xx series chips and would like to know, how to covert LVTTL voltage from Stratix II to 5v TTL and back? 

Any good known way to do this?

[Altera_Forum]

Voltage translators :-)

[Altera_Forum]

--- Quote Start ---  Voltage translators :-)  --- Quote End ---   Which one? I'm newbie here :(

[Altera_Forum]

There are several solutions; 

 1) If your signals are unidirectional, or are bidirectional, but you have a direction control bit, then you can use 5V tolerant buffers; these devices are powered from 3.3V, but can handle 5V on their pins.  If the 5V devices you are communicating are TTL compatible, then these 3.3V translators can work fine.   You need to check the VIH and VIL limits of the 5V logic are met by the buffer.  If you only have a few I/O lines, then look at the TinyLogic devices from Fairchild, and TI (TinyLogic is the Fairchild name). Look in a translator guide and you'll find examples of 8-bit, 16-bit, etc devices.  2) If you have to attach a 3.3V device to a bidirectional bus, and you do not know which direction the signals on the bus are going to be, then you can protect your 3.3V logic using a bus switch. The bus switch will pass the traffic on the 5V bus through itself, and clamp the highs to below 3.3V.   So what is it you need?  Cheers, Dave

[Altera_Forum]

All signals are unidirectional.  

I need to connect 5v TTL to 3.3 LVTTL FPGA and not to burn FPGA input driver accidentally :)

[Altera_Forum]

--- Quote Start ---  All signals are unidirectional.  I need to connect 5v TTL to 3.3 LVTTL FPGA and not to burn FPGA input driver accidentally :)  --- Quote End ---    How many bits?  Pericom 8-bit PI74LPT245, 16-bit PI74LPT16245 are bidir, 244s will be unidirectional.  http://www.pericom.com/pdf/datasheets/pi74lpt16244.pdf  TI, Fairchild and others have parts too. Eg., Fairchild  http://www.fairchildsemi.com/ds/74/74vhc244.pdf  Type in '244' into their parts browsers ... actually, type it into Digikey.  The key thing to look for in the data sheet is to check that the inputs are 5V tolerant when the device is powered from 3.3V. The part numbers will be things like LPT for the Pericom, LVC I think for TI, etc.  Cheers, Dave

[Altera_Forum]

--- Quote Start ---  How many bits?  Pericom 8-bit PI74LPT245, 16-bit PI74LPT16245 are bidir, 244s will be unidirectional.  http://www.pericom.com/pdf/datasheets/pi74lpt16244.pdf  TI, Fairchild and others have parts too. Eg., Fairchild  http://www.fairchildsemi.com/ds/74/74vhc244.pdf  Type in '244' into their parts browsers ... actually, type it into Digikey.  The key thing to look for in the data sheet is to check that the inputs are 5V tolerant when the device is powered from 3.3V. The part numbers will be things like LPT for the Pericom, LVC I think for TI, etc.  Cheers, Dave  --- Quote End ---   Thanks! That is what I really need.

[Altera_Forum]

--- Quote Start ---  Thanks! That is what I really need.  --- Quote End ---  Keep in mind that if you are driving the inputs of the FPGAs with a buffer, that the FPGA pull-ups are no longer any use. It will be the input to the external buffer that determines what logic level is on the FPGA inputs.  So, if your buffer is going to be driven by signals that can be removed, eg., are going to a connector, then you need to put pull-ups or downs on the signals at the input to the buffer so that they do not float when the connector is removed, assuming the FPGA is still powered in that situation. You can save yourself some pull-ups by using a buffer with bus-hold, however, if you want a specific logic level, then using pull-ups and downs allows you to do that.  Be careful with the pull-ups, you do not want to tie them to the 3.3V rail of the buffer, since you will be driving them with 5V and then your I/O pin might try to power your FPGA board if you power it down while the 5V I/O is high. Ideally you should use pull-downs on the buffer inputs.  Cheers, Dave