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UART voltage levels question :)

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Hello guys and gals


Another newbie question :)


I have a device that I want to communicate with over UART using the good old MSP430G2553.


The device will have a supply voltage of about 4V and has an absolute max voltage rating of 3.1V on any input pin including the UART, typical 2.8V. Min. 2.4V for HIGH.


First of all, from what I can read from the datasheet the MSP430 will have a UART logic high of Vcc - 0.3V and I am thinking that since the MSP430 supply voltage range is 1.8-3.6V, would it be "good practice" just to power the MSP430 with let's say 2.8V and thus have compatible UART levels with the other device or should I opt for a logic level converter? Pros cons?


Also in the MSP430 datasheet I noted that there is a relation between supply voltage and system frequency. So will the mcu not run at full speed if I power it at 2.8V? It looks like it would be around 12MHz at that voltage.




Kind regards


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The datasheet also has this nice graph that tells you what the maximum frequencies are, given a certain supply voltage.

If you cannot lower the voltage to the desired operating level for the UART, you could use a logic level converter. You could also use the Vf voltage drop over a diode as a level converter, dropping roughly 0.6 to 0.7 volts per diode in forward setup.

So if you have a supply of 4V, which is too high for the MSP430, you can use one diode to drop it to about 3.3V, which is enough to power it at even 16MHz.

Then you need to drop from 3.3V to 2.4, which is again 0.7V, so you can use a single diode in the UART line from the MSP430 tx to the device's rx. Note that since the device will send out UART at 2.8V typically, you should not drop the level there, while it's still high enough to be considered a high level by the msp430. This is true because the datasheet says that worst case 0.75 Vcc is still considered high, 3.3 * 0.75 = 2.475V it's close, but since the current will be really low, so will the voltage drop (it's not really a constant voltage drop).

So even with worst case parameters, you can play things fine when using only 2 diodes.

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