Hi, On 03/19/2010 10:14 AM, Bas Mevissen wrote: > On Thu, 2010-03-18 at 23:04 +0100, Joerg Albert wrote: >> On 03/15/2010 09:33 AM, Bas Mevissen wrote: >> >>> Do you have access to an oscilloscope? It might be that the signal > level >>> or signal shape is not perfect. I've seen mixed results with various >>> serial to USB adapters too. >> I used an oscilloscope yesterday and the signal looked fine (sharp > edges, correct timing, low noise). > > What were the voltages of both "0"'s and "1"'s?
I attached an oscilloscope today again and saw some strange voltage levels (wonder why I missed them in the first place): If the TX (from the target to the PC) starts after some quiet period, high is at 1.7V and low at -0.6V. Both get slowly better (2.5/0) with some chars transmitted, but the first levels are definitely wrong. I looked closer at the PCB and it turned out that we have a voltage divider with two 5.6 kOhm to V_3_3 and GND (R613, R614) and a capacitor C496 (!) towards the CPU. The signal at the CPU looked fine for a 2.5V TTL. The voltage drift seen above is probably caused by the capacitor unloading when the CPU pin is driven down. I removed the resitors and replaced C496 by a 1k resistor (to protect the CPU pin against shorts). This solved my problem. I guess the above schematics was meant to be a cheap TTL level conversion 2.5V -> 3.3V. Thanks for sending me again to the oscilloscope! Jörg. _______________________________________________ openwrt-devel mailing list [email protected] https://lists.openwrt.org/mailman/listinfo/openwrt-devel
