The problem involved the translator in the subject line. We've used these translators in a number of applications, but in a recent application on a new PCB, I found that one of the four outputs was producing signals above and below a mid-level reference, instead of ground. The critical pin assignments to the part are as follows:
14 (VCCA) - 2.5V; 13 (VCCB) - 3.3V; 8 (GND) - GND; 7 (OE#) - GND; 16 (DIR2) - GND [B-to-A]; 11 (B2) - transmit signal [3.3V ref]; 2 (A2) - receive signal [2.5V ref];
The problem was observed on pin 2 of the translator using an oscilloscope. There was nothing unusual about the transmit signal on pin 11. The problem signal looked like a dual-level signal that would go up and down from a reference I would estimate to be about 1.2V (the difference between the 3.3V and 2.5V rails). The lower level signal went between ground and this mid-level reference. The higher level signal went up from the mid-level reference to some higher level that I did not record. The signals themselves were clean, just not at the correct levels. This problem appeared on both of our two prototype boards tested.
Debug and Work-Around: There was a simple 50ohm controlled-impedance trace between the translator's pin and the receiving device's TDI pin (JTAG). So with little to work with, I cut the trace and measured good 2.5V signal coming out of the translator's pin 2. This told me that the I/O structure of the receiving device's TDI pin was influencing the problem. Since the receiving device is a known good device also used on multiple designs before, I tried the same signals going through the translator in a different direction. So I had the following pin re-assignments made:
14 (VCCA) - 3.3V; 13 (VCCB) - 2.5V; 16 (DIR2) - 3.3V [A-to-B]; 3 (A3) - transmit signal [3.3V ref]; 10 (B3) - receive signal [2.5V ref]; After making this change, the problem was gone and good 2.5V signal was observed coming out of pin 10.
In conclusion, we are using the workaround described above. The question I pose is if there is any good explanantion for this behavior which was conditional on the particular pins, voltages, and receiving device used?
Thank you.
