Other Parts Discussed in Thread: MSP430FR2355
Hi,
I have a design including. the MAX3221. My circuit is built around a 3.3 V supply according to the device datasheet and the reference design (see here). Receiving from a PC host with FTDI cable via MAX3221 to MSP430FR2355 works. Sending to the PC will not: even in idle state (TX signal from MSP430 is high) I observe a heavy oscillation on the driver output of the MAX3221 (see image). This results in the PC reading data which makes no sense at all.
When I disconnect the FTDI cable, the driver output oscillation will not go away completely:
I checked the voltages at the C1/C2 and V+/V- pins. Voltages at C1/C2 seem OK, I observe the 250 kHz oscillation of the charge pump. The voltage at V+ seems ok, too:
So I checked the voltage at V- also:
And this seems really weird to me, because the application note about charge pumps says the ripple at V- would be less than the ripple at V+. I checked the solder joints and even tried a different prototype, but I still observe this. I also increased the capacitor values from 100 nF to 470 nF, which will be OK for supply voltages of 3 - 5.5 V according to the datasheet, but still there's the output oscillation.
I tried a different mode of operation (ForceOn/ForceOff pins), but that doesn't improve anything. It must have something to do with the negative voltage, because when I switch to a "low idle" state (by simply setting the MSP430 output level to zero), I don't see any oscillation, since the RS-232 interface will invert and boost the signal (from 0 to V+, in this case). Do you have any thoughts on this?
