This thread has been locked.
If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.
SN74LVC8T245: 5VTTL to 3.3VTTL bidirectional level shifter: What's the Best choice
Part Number: SN74LVCC3245A
I'm trying to take a 3.3V microcontroller and communicate with some very old legacy 5V TTL signals. Initially, I was tempted to use the SN74LVCC3245A but I noticed through another forum post that it must have a HI level input voltage minimum of 3.5V.
I was able to find some uni-directional level shifters and/or buffers that are able to act as either a bank of inputs or outputs, but cannot be switched. This application requires re-configuring the part to be bi-directional via the microcontroller (using a direction pin) since there are many different confgurations that the system needs to accommodate.
Given these requirements, would you have a part to suggest or perhaps a tweak to the SN74LVCC3245A circuit that would allow it to accept these TTL signals without being overly complex or expensive?
Are the legacy signals 5 V TTL or 5 V CMOS?
Because in the former case, 3.3 V is a valid high voltage, and you can simply run the transceiver with overvoltage-tolerant inputs at 3.3 V (and use a simpler one like the SN74AHC245).
We are glad that we were able to resolve this issue, and will now proceed to close this thread.
If you have further questions related to this thread, you may click "Ask a related question" below. The newly created question will be automatically linked to this question.
In reply to Clemens Ladisch:
Thank you for the speedy reply!
The 3.3V-side is CMOS from a microcontroller which needs to both receive inputs from the 5V TTL legacy signal (an acceptable HI level from the signal source could have a minimum of 2V), and with a flip of a direction bit, output to a 5V CMOS signal (output voltage must be greater than 3.5V, ideally greater than 4.95V). I know it's obnoxious to have to cover all of these cases, but I'm trying to design around industrial systems that could be installed 30 years ago (TTL) and also those that were designed new this past year (CMOS).
I see what you're saying, powering the SN74AHC245 at 3.3V and if it gets hit with an input signal of 5V it doesn't care. However when the direction is flipped the other way it will only output a 3.3V signal which isn't enough for a 5V CMOS system to see a HI.
would there be a part (or couple of parts) that can be used for bi-directional communication between a 3.3V microcontroller and a 5V system that might be either CMOS or TTL?
In reply to Dave Young:
There is no direction-controlled level shifter with TTL-compatible inputs.
So you have to use two devices. For the 3.3 V to 5 V direction, use a normal shifter (e.g., SN74LVC8T245). For the 5V to 3.3 V direction, use any buffer running at 3.3 V with overvoltage-tolerant inputs and with active-high OE pin (e.g., 2× SN74LV126A).
All content and materials on this site are provided "as is". TI and its respective suppliers and providers of content make no representations about the suitability of these materials for any purpose and disclaim all warranties and conditions with regard to these materials, including but not limited to all implied warranties and conditions of merchantability, fitness for a particular purpose, title and non-infringement of any third party intellectual property right. No license, either express or implied, by estoppel or otherwise, is granted by TI. Use of the information on this site may require a license from a third party, or a license from TI.
TI is a global semiconductor design and manufacturing company. Innovate with 100,000+ analog ICs andembedded processors, along with software, tools and the industry’s largest sales/support staff.