Fast bidirectional 3.3 to 5V level shifter

Question

For a hobby project of mine, I'm thinking about connecting some old 5V SRAM chips like this one to a 3.3V - capable FPGA. The target frequency is 50MHz. After some simulator experiments I found out that a simple voltage divider could work:

^{simulate this circuit – Schematic created using CircuitLab}

This gives a nice 0.2..3.0V peak to peak signal on the 3.3V side when driven by 5V side. In the opposite direction however, the output signal is only 0.4..2.6V peak to peak, which is kind of within the spec for a W24512AK (\$V_{0H}=0.8V\$ and \$V_{1L}=2.2V\$) but the margin isn't great. I wonder if this will work at all in practice.

So I'd like to improve the margin by using an active level converter, but as far as I can see, all simple schematics (e.g. the ones listed here) have a terrible frequency response. The only viable solution I see is to buy some TXB0108 which can reach 100Mbps at 3.3V.

Is there a level converter schematic I have overlooked? As such, it doesn't have to be truly bidirectional since the \$\overline{WR}\$ signal is accessible and can be used to enable/disable shifters in each direction.

Answer 1

I am slightly skeptical on your architecture. I used ALVC256 chip for the 3V3-5V conversions, and even with this would-be-correct architecture there're several issues which, at some circumstances, may make device malfunction.

Consider the following:

• 50 MHz is 20 ns cycle, SRAM you are going to use is 15 ns read/write cycle. You have slack of only 5 ns to ensure that buses (address and data) are stable and select proper memory cell for access;
• Mismatching impedance will cause spikes and false positives at the SRAM side, killing your 5 ns slack and potentially accessing (reading/writing) wrong data. This may require very fine tuning to work properly under all conditions (e.g. temperature, power source condition).

If you want well working device, then

• Get 3V3 SRAM, and connect it to the FPGA without any converters using series resistors (which you can change to tune for impedance);
• OR relax frequency conditions down to, let's say, 20 MHz (50 ns cycle).

Answer 2

• 3.3V → 5V:
  a 74HCT244 powered by 5V (s. datasheet: min. High level input voltage) should work.
• 5V → 3.3V:
  a 74HCT244 powered by 3.3V and series input resistors to limit input current via protection diodes.

This solution needs each buffer enabled for each direction (as you have addressed in your last sentence).