| ??? 04/02/12 12:13 Read: times Msg Score: +1 +1 Good Answer/Helpful |
#186972 - FTDI chip ESD sensitivity Responding to: ???'s previous message |
I've just designed an FT232RL into a product and it took a lot of effort to get acceptable ESD immunity. Your symptoms sound similar, so I would have a look in this area. USB links horrible from an ESD viewpoint because you can spark to the end point and the ESD spike can travel through the USB cable to the PC in preference to a local earth.
In the end my solution involved transient suppressors on the USB 5V, DM and DP lines and opto-isolators (actually ISO7221AD chips) on the UART TX and RX signals. The FT232RL was USB powered, so you don't need an extra isolated 5V supply. I did try common mode chokes between the FT232RL and my CPU (instead of opto-isolators) and they had some effect, but not enough to meet CE levels. The idea was to add impedance into the ground loop, therefore controlling where the ESD goes. I might have got an acceptable result with common mode chokes if I'd spent more time on it, but I didn't have the time. Connecting the USB screen to the correct ground is vital, as you get quite different behaviour if it's the CPU 0V, USB 0V of the chassis metalwork. |
