Number of 74HC595 on one bus?

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Hi

I have been using 8 units of 74HC595 on one bus driven by the PIC16F873. This works fine.

I then put on another 74HCT595 bringing the total 74HC595's to 9, and things go very wrong.

Could some one please tell me if there is a limit as to the number of 74HC595 units that can be used on one bus?

Thank you
a.

 

[audioguru]

Cmos doesn't have any input current so the limit on how many can be on one buss is their input and wiring capacitance and the frequency, or narrowness of the pulses.

 

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Hi AudioGuru

Thank you for your reply.
So there is no limit of 8 74HC595s on one bus?
How can I test if I have a capacitance problem?

Regards
a.

 

[audioguru]

How can I test if I have a capacitance problem?

Reduce the frequency of the data's rate, then add a 9th 74HC595. Increase the frequency of the data's rate until it doesn't work anymore. Remove the 9th and the system will work fine.

 

[Oznog]

You may have other problems.
Do the 74HC595 have appropriate bypass capacitance on the power rails? If you switch all those together then it makes a fairly hard current spike and that can cause logic errors or scramble the PIC.

 

[justDIY]

along the same lines as oznog's recommendation ... and with a bit of wild assumption thrown in for good measure

if you're switching LEDs or some other "load" with your 595's, does your main power supply have enough capacity to handle it? Maybe the load on your 9th 595 is the straw breaking the back (exceeding the capacity) of your power supply?