MAX233A component not working

[Integrol]

Hello,

In my project, I m using a MAX233A to convert a RS232 signal to UART to control a microcontroller.

I generate my RS232 signal with a usb-rs232 cable and using Putty.
This is what I generate by sending the "j" character.



I use a MAX233A as below:



So I look at the UART signal generated by the component: ( letter j)



The signal should be TTL and the LOW level is about 1V and Hight level 3.7 V with a big oscillation.
I checked all the grounds, they seem to be good. The supply voltage is 4.8 V.

Do you have any idea what's going on?

Thank you

[Manul]

"Big oscillation" is likely a charge pump ripple (although it is quite high, might indicate heavy loading). The main problem is levels. Is R1OUT loaded by something? Connected to MCU? Or is it unloaded?

It kinda looks, that it is connected to 3.3V CMOS output driven high and fights that. Probably you wouldn't do such mistake. Fake IC? What is idle current consumption of your MAX233A?

[Integrol]

Yes, R1OUT is connected to my MCU ( RX port ). But even unloaded, the result remaine the same because I use a picoscope to capture my signal ( input impedance of 10 MOhm ).

I don't think it's a fake IC because I bought this component from Mouser. The power consumption is a few mA.

I checked all the voltages of the charge pump ( V+, V- , .. ) and everything seems to be fine.

[Manul]

Yeah, I don't know, schematic looks ok to me. Just for the peace of mind I would disconnect the R1OUT from MCU (cut the trace or something).

[macboy]

Where are the charge pump capacitors? It seems you just shorted the pins together.
Edit:
Google-fu failed me, the 233 has internal charge pump caps apparently. Carry on.

[Integrol]

Manul : So you were right, I cut the line between the MCU and the output of the MAX233A and I have TTL with 0 V for low level et 5 V for high level.
It is when I connect the MCU that I have a problem.

My MCU is a C8051f850 powered by 3.3 v so yes, maybe the UART input does not accept TTL level but 3.3 V level only.

macboy : Yes no capacitor needed

[Manul]

Yeah, so the problem was exactly how it looked that it was. And also as I said, your "UART input" is likely configured as output and driven to 3.3V. Or the input is somehow damaged and stuck at 3.3V. I'm saying this because your low level was also wrong, looked like a fight between CMOS output sourcing current and MAX233A trying to pull it down (and keeping in mind that low side mosfet is often stronger in cmos, 1V resultant level is about realistic). So two things:

1. You can not just connect 5V levels to 3.3V MCU if it is not "5V tolerant".
2. MCU input is misconfigured, damaged or not a UART RX pin (mixed RX and TX?).