1.8v to 3.3v level shifting (small footprint, low power & cheap)

[clem]

I'm working on a small (the board is under 30x30mm) very low (average) power design. One part on the board has 5 input control pins (MOSI, SCLK, CS, +2 others) at 3.3V (@ ~1.5mhz max), however the CPU (a cortex-m0) has 1.8V IOs.

So I'm trying to find a small footprint (but no BGAs), low power & cheap way (choose any two?) to accomplish this.

A digikey search turned up many different level shifter parts to solve this, but they all seem to be overly expensive for the functionality (the CPU is < US$2.50). Parts like the 74LVC8T245 or 74LVCH8T245 would do the trick, but the DHVQFN24 is a bit large at 3.5x5.5mm and it seems to be ~US$0.65 Q100.

I'm only translating in one direction (low to high) so I don't need a bi-directional translator, or output enables. I would have thought the cheaper solution would be a small/cheap octal non-inverting buffer that can accept 1.8v inputs with a 3.3V supply. But the cheap buffers tend to have a Vhi of  ~2-2.2V and the cheap parts where all in monstrous TSSOP packages.

Any ideas? Space and power are a premium, but at the same time I don't want to spend 30% of the CPU cost to shift 5 signals 1.5V.

                               --Clem

[ivaylo]

You are probably aware but a simple way of doing it is this - http://www.adafruit.com/products/757. It is two way and you say you don't need it but it's tiny, probably cheap in quantity, etc.

[T3sl4co1l]

A discrete solution needn't be very large (a couple SC-70-6 transistor pairs and fine pitch resistor networks?), but the speed and power consumption of open collector/drain outputs tends to be poor.  I wouldn't suggest it for battery operation and signals over 1MHz.

Offhand I don't know of any level translators like that.  There's a bunch of those damn SN74CBDHVCOMGWTFBBQ60245XYZs out there, in as many alphabet-soup-FN packages, but I don't think I've seen any in low bit widths.

Don't suppose you can convince one party or the other (the MCU or peripheral) to operate on something else.  Say, what's an M0 doing at 1.8V anyway?  There are oodles of those available in 3.3V.  If you're so strapped on current that you absolutely must use a low voltage part (I assume the power specs on it are better than most?), you're probably equally well off reconsidering the codebase.. I'm sure heavy optimizations can be done to facilitate at least as much power savings, for at least as much bother as fixing it in hardware.

Tim

[David Hess]

This is easy to do with transistors but low power cuts against open collector/drain outputs and simple circuits may have noise margin issues.  3.3 volt input tolerant buffers or inverters can be used as level shifters.

Depending on your speed requirements, analog comparators can handle this easily with a minimum of power and space requirements.  Just be sure to use ones that have push-pull outputs.

[senso]

If its just inputs, why not use just a simple resistor divider to get those 3.3v down to 1.8?

[clem]

Offhand I don't know of any level translators like that.  There's a bunch of those damn SN74CBDHVCOMGWTFBBQ60245XYZs out there, in as many alphabet-soup-FN packages, but I don't think I've seen any in low bit widths.

Yeah I was swimming in a sea of choices, but none quite did what I wanted or where too expensive for such little functionality. I was hoping someone here came across this problem and could point out a cheap solution.

Don't suppose you can convince one party or the other (the MCU or peripheral) to operate on something else.  Say, what's an M0 doing at 1.8V anyway?  There are oodles of those available in 3.3V.

The lower the core voltage the lower the power consumption, because the switching power is C * V^2 * f. So lowering the voltage has a bigger impact then lowering the frequency. It's why larger processors can directly control their own core voltage, so they can run the part at the lowest voltage for a given frequency (and temperature, etc.). The Cortex-M0 part can run at 3.3V but running at a lower core (and IO) voltage is an easy way to save power.

This is my first foray into very-low power (micro watts) parts. I've worked on larger devices where you put the interrupt handlers in SRAM (or lock them in the icache) so you can put the DRAM in self-refresh mode and power most of the IOs, DLLs etc. But never something where every microwatt counts. It seems like it is another world.

When the system goes idle I turn off the 3.3V supply and put the regulator in a low power mode. The CPU switches off the main clock and runs off of a 32khz clock for timers. They even have a mode where you turn the entire part off except for a small bit that can turn the processor back as if from reset.

I'm mostly using components that run at 1.8V or have split VDD/VDDIO rails. All the wakeup events come from devices on the 1.8V supply. However the LCD (a Sharp Memory Display) has a 3.0V minimum supply and a minimum Vhi of VDD-0.1V so there isn't any room there. I wonder if the entire control interface on the display is implemented in transistors on glass or if there is a bit of silicon hiding someplace. Transistors on glass aren't know for being overly fast or low voltage.

[miguelvp]

There is the old philips application note for I2C level shifting

http://www.adafruit.com/datasheets/an97055.pdf

I believe there is an updated one by NXP but pretty much is the same thing but I can't find it.
This only deals up to 400kbps not sure how it will behave at higher rates. And it's bidirectional.

Not sure if it will fit your low power small footprint.

[rxxbxyhxxb]

http://www.nxp.com/documents/data_sheet/74LVC07A_Q100.pdf

I hope it could help you.

[Jeroen3]

While searching for such a relatively simple part it is best to look at the websites of manufacturers themselves. Most of the time they are better organised than the farnell or digikey database.

[nsayer]

In the "downward" (3.3 -> 1. direction, my first inclination is a reverse-biased diode and a pull-up resistor. In the "upward" direction, I'd check to see if maybe the lower levels were still high enough to work - 1.8 volts is more than 1.5, so it's conceivable that your 3.3 volt might be able to take the 1.8 volt input directly.