Hi,
A customer of mine is designing an FPGA board that has to interface to a VME bus.
The FPGA has 1V8 interfacing levels but the VME bus works at 5V levels.
For that SN74LVC level translators seem to fit the bill for level translation between 1V8 and 5V levels
What I cannot derive is whethter the drive strength and hi-low levels of these level shifters will be compliant with the VME standard.
Do you know if that will work?
Hi Jan,
The VME standard, IEEE 1014-1987, mentions that TTL levels are used - meaning that the thresholds are as shown on the left in this image:
The vast majority of devices in use today operate on the right hand scale, "5-V CMOS" due to the speed, low power, and low cost of CMOS devices. Note that a signal output from a 5V TTL device will not always be within the 5V CMOS input specs. This will depend heavily on what parts are being used in their system -- is the VME bus truly operating at TTL levels (ie using antiquated BJT logic), or is it using more modern CMOS devices to operate? This is not governed by the standard (from what I could find), but is a choice each manufacturer makes when designing with the VME bus. The term "TTL" has become loosely interpreted as "5V logic" by many.
Additionally, the standard mentions that "the pcb designer" should "study transmission-line theory, and certain specific parameters of drivers and receivers that are not normally found in most manufacturers' data sheet." It doesn't really say what the bus's maximum capacitance on each line should be. Again, this is left up to the designer.
In my opinion, LVC is a good choice for this application (as a strong 5V driver), however I would have to know more about their system to give a good answer as to whether it will definitely work or not.
Hello Jan,
While I am not an expert on the VME bus standard specification, I tried looking through some of the available documentation and found the following:
1. Voltage Thresholds - i have seen that it works at both 3.3V and 5V, but thois is what i found from a document online.
Given this specification, our device powered from 5V will be able to meet the voltage thresholds properly.
2. Drive strength:
From what it looks like below, the LVC device will not be able to meet this specification, as it is rated for 32 mA maximum.
Please note that we do have a VME bus driver device:
http://www.ti.com/product/SN74VMEH22501A
This device should meet the necessary specifications for driving a VME bus (although it is a 3.3V, which I am not sure if it is acceptable and it also cannot up translate from 1.8V).
There is also an app note for this device which may be helpful for the customer to review.
http://www.ti.com/lit/an/scea028/scea028.pdf
I hope this helps, let me know if you have additional questions.
Best,
Michael
