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Hi, I bought a very cheap clone with “USbee” stamped on it and thanks to the great info here I modified it to a Saleae clone. Also my device has a 74HC245 buffer at the inputs and I discovered that this is not a nice solution. This 74HC245 chip is powered with 3.3V coming from a 3.3V regulator. So far nothing wrong with this, HC technology can be powered from 2 to 6 volts and the buffer will provide (a little) ESD protection for the inputs. So what is my point?

Well, this chip will work O.K when you connect its inputs to a DUT with 3.3V logic. But connecting it to a DUT which has 5V logic can give some serious problems. When I connected one of the inputs to a 5V high signal (’1′), I saw the analyzer taking a current of 7mA out of my 5V source!! This is not what you want: you want an analyzer with “high impedance inputs” for all signals from 0 – 5V!

The cause is the buffer being a HC type. When HC technology is powered from 3.3V one should NOT feed the inputs with a 5V level! When using HC technology, the maximum input signal level should NOT exceed Vcc + 0.5V! (see specs ) If you ignore this, the chip will take a lot of current and can break down. In my analyzer the current is limited because series resistors of 100 ohms were used. So the chip will not break down, but we can not call this “5V tolerant inputs” because it will act as a heavy load for our connected DUT when signals rise above 3.3V. ( note that the Cypress microcontroller ITSELF is 5V tolerant!

) So for a DUT with 3.3V logic levels the analyzer is O.K, for a DUT with 5V logic it will NOT offer high impedance inputs when input levels come above 3.3V! There are some solutions for this problem. Remove the 74HC245 and connect its inputpads to the output pads. This is an option because the Cypress microcontroller itself is 5V tolerant. But now the pins of the microcontroller are directly coupled to the world outside with no kind of protection at all. Maybe not a good idea.

You can modify the pcb tracks in such way that the 74HC245 will be powered from the 5V USB voltage. This will work O.K when connected to a 5V logic DUT, but now in contrast we can get in trouble when we connect a DUT with 3.3V logic: the level of the high (’1′) signal from the 3.3V DUT is maybe not always quite high enough now for being a logic ‘1’ for the 74HC245 inputs. Not so reliable. But adding a select switch to choose the Vcc of the existing 74HC245 chip between 5V of the USB or 3.3V from the regulator could be an option.

But the housing is very small and maybe you have to put the PCB in a larger box then. You can connect an extra 5V tolerant buffer chip to the inputs of the analyzer. This can be a 74LCX245 or equivalent, which is 5V tolerant with its Vcc connected to the 3.3V regulator. You will need a bigger box to add this extra chip and a new connector to the testleads. You will have extra timedelay at the inputs which is not wanted, but probably will not give serious problems. You can REPLACE the 74HC245 chip with a 74LCX245 or equivalent, which is 5V tolerant when its Vcc = 3.3V This will be the best solution using the original box now and the analyzer will have nice high impedance inputs for ALL signal levels from 0 to 5V.