Problem with TC7107 LED Display Driver

[king.oslo]

Hello there

I've got a TC7107 wired like in the "typical application" in the data sheet.

The only modifications I made is to use different Vref+ and Vref - (+2.5V and 0V), and I changed the RC values of the RC oscillator to 680pF and 3.3kOhm (I should get crica 200.53kHz)

The problem is that the 3 1/2 display is displaying: "1    ". Basically a "1" in the 1/2 segment, and nothing in the other three. The LEDs are wired correctly, because when I pull the TEST pin high, all segments light up.

I tried to measure the legs of capacitator with a scope with respect to GND. I expected to see a waveform, but I cannot see one, other than perhaps some noise.

How do I go about troubleshooting the TC7107 to find the fault?

Datasheet can be found here:

http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en010503

Thanks.

Kind regards,
Marius

[david77]

Are you shure it's not just indicating overflow? The classic ICL7107 do that by displaying "1".

[Balaur]

Assuming that everything is connected correctly, it looks to me that the clock is not starting.

The only modifications I made is to use different Vref+ and Vref - (+2.5V and 0V), and I changed the RC values of the RC oscillator to 680pF and 3.3kOhm (I should get crica 200.53kHz)

I'm very sorry to be a bit condescending, but judging by your previous questions on this subject, I cannot help myself to not ask you:

Which part of the following sentence of the datasheet have you not understood:

ROSC (Pin 40 to Pin 39) should be 100 k.

You have used 3K3. Surely, there is a bit o discrepancy, don't you think?

Please try to implement the recommended schematic to verify that everything works correctly, then you'll have all the time you want to be creative.

Cheers,
Dan

[king.oslo]

-

[king.oslo]

Are you shure it's not just indicating overflow? The classic ICL7107 do that by displaying "1".

David, I don't know.

Voltage between Vref pins are 2.5V. Should give a range of 5V across the 2000, counts, right?

Voltage between In+ and In- are 0.48V, so I expected the display to say 192

Do you agree with this?

Assuming that everything is connected correctly, it looks to me that the clock is not starting.

I'm very sorry to be a bit condescending, but judging by your previous questions on this subject, I cannot help myself to not ask you:

Which part of the following sentence of the datasheet have you not understood:

ROSC (Pin 40 to Pin 39) should be 100 k.

You have used 3K3. Surely, there is a bit o discrepancy, don't you think?

Please try to implement the recommended schematic to verify that everything works correctly, then you'll have all the time you want to be creative.

Cheers,
Dan

Thanks Dan,

I did what you said. Still no waveform between the capacitator and GND. As said previously, there is something that looks like noise, however. The amplitudes are about 10mV, so I expect that it isn't the clock signal?

Thanks.M

[Balaur]

The clock signal should be observed on OSC3.

You could try the following:
- zero the input voltage (i.e. connect IN HI with IN LO): the indication should be 0000.
- pull the "TEST" (connect it to V+). All the led segments should be lit

If none of these checks work, and assuming that the circuit is implemented correctly, a (very unlikely) explanation could be that somehow the oscillator buffers have been damaged by the low Rosc.

[king.oslo]

The clock signal should be observed on OSC3.

- There is nothing there.

- zero the input voltage (i.e. connect IN HI with IN LO): the indication should be 0000.

If I connect them together, the display still says "1   ".

- pull the "TEST" (connect it to V+). All the led segments should be lit

This works as intended. This displayes "1888".

Once I get some 100pF caps, I will solder everything exactly like the datasheet suggests, and see if that helps.

Unless you have some more thoughts.

Thanks.

Kind regards,
Marius