Feature request: Adjustable threshold voltage

[leonvs]

As you can imagine, Digilent equipment is often used in noisy test environments. Often these environments are sensitive to crosstalk, especially on the wires to the test equipment. Especially when multiple lines change their logical level at the same moment, this easily leads to crosstalk on other lines influencing the stability of the (digital) measurement.

To (partly) deal with this it would be very helpful if the threshold voltage was adjustable. Working in a 3V3 environment setting the threshold to 1V8 or sometimes even 2V solves many of the crosstalk issues influencing the measurement.

So my question is, is it possible to make the threshold adjustable since now it is fixed, depending on the voltage level used.

Also I found that working at 3V3, the threshold for the Digital Discovery is 1V42 while for the ADP3450 this is 1V3. At least for different equipment the threshold value should be the same where 1V3 is quite low.

[leonvs]

I must admit this surprises me a little bit. I work for a large engineering company and very often promote the digital discovery. The thing is that most colleagues react with 'ok, another logic analyzer, boring' I spend a lot of time explaining it is so much more than that. But you know how it works, they stick to the bulky devices they know and use large waveform generators to even generate the most basic signals. For some reason they just don't believe a $200 device with a USB connector can do the same and much much more. Often custom test setups are created to analyze SPI or I2c chips which the discovery can do 'out of the box'. I think that the digital discovery is not positioned right from a commercial perspective. It is so much more than a logic analyzer....... 

Two flaws corrected and the device is perfect, no with a capital P, Perfect. 

Most important, the threshold voltage. This must be adjustable for the device to play in the premiere league. 

Second, a more flexible trigger system with combinations of the basic trigger facilities. 

A new version offering this would be so welcome and make the device Perfect

[attila]

Hi @leonvs

So far, the DIO input logic threshold on all our T&M devices is given by the VIO voltage.

Shielded or ground-twisted signal lines should be used to reduce crosstalk, such as:
https://digilent.com/reference/test-and-measurement/digital-discovery/reference-manual?redirect=1#accessories

[leonvs]

Hello Attila,

Thanks for your reply, I know, the documentation for the Digital Discovery says: threshold voltage is about 0.45*VCCIO_PROG which correspond to the values mentioned in my first post.

Still for the ADP3450 the threshold value when using 3V3 is 1V3 which is not the same as for the Digital Discovery. Because of this the  logic analyzer in the ADP3450 is more sensitive to crosstalk induced glitches.

 

My question is whether the threshold voltage can be made such that it can be set independent from the VIO voltage. So when using a VIO of 3V3, it would be very nice to be allowed to set the threshold voltage over the entire range from 0V to 3V3.

Most competing products offer this to adjust to the environment under test.

 

[attila]

Hi @leonvs

Threshold voltage cannot be set independently of VIO. The threshold value is set by the supply voltage (VIO) of the FPGA bank.

The threshold and hysteresis difference is likely due to the device's FPGA IOs.
Digital Discovery uses Spartan6 and ADP3450 Zynq (Artix7). DIO frontends are similar, see:

https://digilent.com/reference/test-and-measurement/digital-discovery/reference-manual#io_level_translators

[attila]

Hi @leonvs

You could eventually use the DIO/DIN pull up-down to shift the input signals a bit, by about 0.4%/10%

[Doc TB]

I was about to create a new thread when I found this. I totally agree with OP. The Digital Discovery is a very nice tool and the software is good enough for many applications, but not being able to set thresholds voltage where all other similar tools can adjust it (usually by 0.1V steps) is a deal breaker. As OP said, when working with 3.3V levels, the crosstalk add so many noise (even at low frequencies) that the whole tool is unusable. On other LAs, I usually push the threshold to 2V or 2.2V and glitches are gone. Here I have no solutions: enabling pull up/down doesn't help and using twisted wires is not possible due to the lack of enough ground pins on the debugging adapter.

Is a new revision (eg: Digital Discovery 2) planned that could add this feature? Is there any other Digilent tool that can work as LA with adjustable threshold? Would be happy to buy it.

[attila]

Hi @Doc TB

Thank you for the feedback.

There are currently no plans for a new generation of this device.

The threshold is adjustable up to 2V with ADP5250 (VirtualBench), but this is a different class of device, 1GHz scope, supplies, dmm and less special features.

[attila]

Hi @leonvs

Thank you for your feedback.

[MirceaDabacan]

On 12/5/2023 at 2:14 PM, Doc TB said:

using twisted wires is not possible due to the lack of enough ground pins on the debugging adapter.

Hi All, @Doc TB, @leonvs

For the 24 input only pins of the Digital Discovery, Digilent offers the High Speed Adapter and Logic Probes. This is a small PCB adapter which extends the number of GND pins to 24, one for each DIN. Each Logic probe is a set of twisted wires, (black for GND, red for DIN), with a 100ohm series resistor on the top of the red wire (for reducing reflections). This solution reduces crosstalk and enables high frequency signal acquisition.

https://digilent.com/shop/digital-discovery-high-speed-adapter-and-logic-probes/

Edited December 7, 2023 by MirceaDabacan