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SN74LVC1G74: CLR held low until PRESET goes high

buckshot
Prodigy 110 points
Part Number: SN74LVC1G74

Hi,

I've got a weird issue with the SN74LVCG74 on initialization. I have a comparator pulling the PRESET line low for a couple of milliseconds, while a pullup resistor is pulling the CLR line high (or at least trying to pull it high). However, I am witnessing both the CLR and PRESET line being held low until the PRESET line goes high. I was wondering if anyone had any idea as to what is happening. 

Again, on bootup, the PRESET line is held low by a pull down comparator for a couple of milliseconds, and then it it released. The CLR signal is just pulled up to 5V with a 20K resistor. However, the CLR line never goes high until the PRESET goes high. I use the CLR line eventually, so I need the pullup resistor there. 

Any Ideas would be great.

Thanks
  • 0
    TI__Guru* 97046 points
    Hi buckshot,

    There is no internal connection between these two - it's likely that there's something external to this device that's causing the issue. What is driving the CLR line eventually? Can you disconnect this device and verify that the CLR line is pulled high through the pull-up as expected?
  • 0 in reply to Emrys Maier
    Prodigy 110 points
    Hi,

    We just realized we're pulling these inputs -5V below ground, which could be the problem. I just want to verify that having the PRESET line held low on boot up and the CLR line pulled up will cause the output "Q" to go high, without having to release and then pull down the PRESET to trigger the output "Q" high.

    Thanks
  • 0 in reply to buckshot
    TI__Guru* 97046 points
    What you have described is correct, and is how we recommend initializing this type of part in a system.

    Most logic devices will be ground referenced and will be damaged by applying a -5V input to them. If -5V is a required input, the best solution is to add a series resistor to limit the current through the negative clamp diode on the device's input (current needs to be limited to the maximums listed in the datasheet: I_IK = 50mA and I_OK = 50mA, total current cannot exceed 100mA)
  • 0 in reply to Emrys Maier
    Prodigy 110 points
    After correcting my error (now applying ground instead of -5V), the chip works as you explained. Thank you very much for your help.