Hi folks,
For many years, I have used a standard D type FF (74x74) to divide an incoming frequency. Hook the D input to the Q output, & works great.
I just used one of the single section D-flops (see title & it's a little bitty part), and it didn't work.
Both the Pre & Clr lines are tied directly to 3.3v, and the supply has good bypassing.
To make it work, I had to put a 10k resistor in place of the D-Q connection and add some capacitance to the D side. (slowing the signal down).
I substituted a standard 74HC74 in it's place (kludge on the board), and it worked fine without the extra circuitry.
Am I seeing some kind of race condition due to the higher speed of the part?
Looking at the data sheet, you have to hold data on the D pin at least 1.2ns after the clock rises, and the prop-delay through the part is 2.2ns. So, at least spec-wise, it looks like I have 1ns to 'spare'.
Ideas?
Thanks,
Tim
For many years, I have used a standard D type FF (74x74) to divide an incoming frequency. Hook the D input to the Q output, & works great.
I just used one of the single section D-flops (see title & it's a little bitty part), and it didn't work.
Both the Pre & Clr lines are tied directly to 3.3v, and the supply has good bypassing.
To make it work, I had to put a 10k resistor in place of the D-Q connection and add some capacitance to the D side. (slowing the signal down).
I substituted a standard 74HC74 in it's place (kludge on the board), and it worked fine without the extra circuitry.
Am I seeing some kind of race condition due to the higher speed of the part?
Looking at the data sheet, you have to hold data on the D pin at least 1.2ns after the clock rises, and the prop-delay through the part is 2.2ns. So, at least spec-wise, it looks like I have 1ns to 'spare'.
Ideas?
Thanks,
Tim