CD74HC237: Problem with switching output (blinking)

Miguel Amezcua Peregrina

Part Number: CD74HC237
Other Parts Discussed in Thread: ULN2003A

I'm using 4 CD74HC237 to send the signals for 6 stepper motors. I have two questions. The first: according to the datasheet, I should be able to use it at 3.3V right?

the second question:

As you can see in the image attached, (there are 4 demux), the An inputs are the same, since I need to open the 4 channels at once.

The OE1 and OE0 are connected to GND and VCC. I keep the selector activated and then LE blinks. The problem is that the output is not blinking. I already checked with the oscilloscope and everything works fine on the MCU. The inputs for the demux work fine, the problem are the outputs...

Here is the table for the stepper motors:

int step[ 8 ][ 4 ] =
   {  {1, 0, 0, 0},
      {1, 1, 0, 0},
      {0, 1, 0, 0},
      {0, 1, 1, 0},
      {0, 0, 1, 0},
      {0, 0, 1, 1},
      {0, 0, 0, 1},
      {1, 0, 0, 1}
   };

Did I miss anything?

Thanks!!!

over 7 years ago

0 Emrys Maier over 7 years ago

TI__Guru* 97436 points

Hi Miguel,
(1) Yes, the recommended operating voltage range of this device is 2V (min) to 6V (max). 3.3V is between those two values, so it is a valid operating voltage.

(2) Are you trying to directly drive motors with this device or are you just sending signals to a motor driver? This device doesn't have much drive current (VOH/VOL are rated at ~4mA).

0 Emrys Maier over 7 years ago in reply to Emrys Maier

TI__Guru* 97436 points

Hi Miguel,
I haven't heard back from you in a while. I'm going to go ahead and mark this issue as 'resolved' -- if you need additional help, please respond to this thread and we can work with you to find a solution.

0 Miguel Amezcua Peregrina over 7 years ago in reply to Emrys Maier

Genius 4015 points

Hi!!

Sorry for the late response. For sime reason I never got the email back with your answer

1) Great

2) no. Im using the demux to send the control signals to the 6 uln2003A.

I forgot. I solved the problem. I was using the LE pin as input signal (control signal) instead of the output enablers. The datasheet is confusing there since the truth table starts with LE and not with the output enablers.

0 Miguel Amezcua Peregrina over 7 years ago in reply to Emrys Maier

Genius 4015 points

I only have one last question. I was unable to find the information on the datasheet. What is the input current per pin???

Thanks

0 Emrys Maier over 7 years ago in reply to Miguel Amezcua Peregrina

TI__Guru* 97436 points

Hi Miguel,
The input current per pin is listed as "Input Leakage Current" or "I_I" in the electrical characteristics table. For this device, the maximum at 25C is 0.1uA, and across temperature it's 1uA.

For most CMOS devices, you can expect the typical value to be in the low nA range.

0 Miguel Amezcua Peregrina over 7 years ago in reply to Emrys Maier

Genius 4015 points

I was wondering, this component only supplies 4mA per pin right? Can I sink enough current to turn on an LED of 10mA with a pull up resistor???

It would work on 0's right??

thanks

0 Emrys Maier over 7 years ago in reply to Miguel Amezcua Peregrina

TI__Guru* 97436 points

Hi Miguel,
You may want to review Kirchhoff's Current Law (KCL).

If you have 10mA flowing into a node (ie an LED), then that 10mA must flow out somewhere. In this case, it would be sinking into the pin that only supports 4mA.

Are you sure it only supports 4mA though? What's the rated maximum in the datasheet? What does that 4mA really mean?

0 Miguel Amezcua Peregrina over 7 years ago in reply to Emrys Maier

Genius 4015 points

It can supply 4mA but I dont saw the sink value. I think is 25mA

I solved that by using a different kind of LED

Thanks!!!

0 Emrys Maier over 7 years ago in reply to Miguel Amezcua Peregrina

TI__Guru* 97436 points

No problem. If this issue is resolved, please click the green "this resolved my issue" button so others with similar questions can find the right answers faster.

Thanks for marking this resolved. Come back any time you need help :)

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CD74HC237: Problem with switching output (blinking)