View Full Version : 74HC165 vs 74HCT165 Shift registers and BCD Thumb-Wheel.
02-13-2007, 09:26 AM
Further to my success with the SHIFT REGISTERS I have another question
According to this article I found..
http://www.proteanlogic.com/applications/An043_old.pdf PAGE 4
There are two types of 74xx165 Shift Registers
the 74HC165 and the 74HCT165.
I believe the HCT version does not require pull-up resistors, so I could ( according to that PDF ) connect directly to the two BCD thumb-wheels without any resistors.
Is this the case?
02-13-2007, 09:43 AM
The HC series devices use an input circuit that switches around the 1/2 way point between ground and the supply voltage (roughly 2V if you power the device off +3.3V). The HCT series has a threshold of around 1.5V when the device is run off +5V.
The HCT series is useful when interfacing with +5V parts where the Propeller is driving the HCT device and the HC device runs at +5V. The HCT series is also useful when the HCT device is run off +3.3V and outputs directly to a Propeller input. None of this has anything to do with pull-ups on the inputs to the 74HCT165. You will need them. A useful value is somewhere between 4.7K and 10K.
02-13-2007, 10:57 AM
I am still a little confused,
Why then does it mention in the PDF link a posted that resistors are not required? PAGE 4 ,
I am referring to the BCD device connecting to the SHIFT REGISTER.
02-13-2007, 11:28 AM
The article that you have is wrong. You do need the pullups. I carefully checked the Philips documentation on HC and HCT devices and there is nothing in the design of the input circuitry that would eliminate the need for pullups. If you leave them floating, the input circuitry's power consumption will increase and the inputs will become very susceptible to electrical noise since they're very high impedance and they'll tend to drift across the linear region of the input CMOS transistors.
02-13-2007, 11:43 AM
Thanks for that. I will dispose of that document!