CMOS 4174 Hex D-type Flip Flop clock reset and input driven from Teensy2.0++

[j303]

I hope posting about an older board is ok here.

I am working on simply understanding how the RE-303 (TB303 replica) creates control voltages from the note information being output from the CPU pins to a Hex D type Flip flop.





the original CPU is a upd650 with a mask rom... anyway, I am trying to output just a middle C, 0x17 from the 6 pins of a teensy for six bits (010111).

I have the code working to output 010111 from six pins on the teensy, but when I connect the output of the teensy pins to a the data input pins of the CD74AC174, I can not seem to get the latching to work from the clock.
For my simple test I connected one pin of the teensy-> PD6 to the 4174 D0 pin and the output of Q0 to an LED. I connected the reset and data in pins of the 4174 to ports that are set to the same output group as PD0, meaning , they all go high at the same time. I do see the LED blink with a delay in relation to the blinking teensy LED, my issue is however that the blinking LED I have connected to the 4174 only blinks when I touch the Reset pin or the data in pin, in a quasi random way sometimes my finger, sometimes the desk... I can't tell why the output pin is being driven high only when the data pin is temporarily connected - even if the jumper is not connected to anything. Just touching the pin activates the output then it stops after a few flashes. Should the clock be driven by an entirely separate clock output from the teensy - like a dedicated clock pin (does that exist) - should my Data in pulse be totally different in time to my reset?

PD0 ->1 ->IC9,4 ->CPU 8
PD1 ->2 ->IC9,13 ->CPU 9
PD2 ->3 ->IC9,3 ->CPU 10
PD3 ->4 ->IC9,14 ->CPU 11
PF0 ->5 ->IC9,6 ->CPU 16
PF1 ->6 ->IC9,11 ->CPU 17
PI 1 ->8 ->IC9,9 ->CPU 31 Slide
PE0 ->7 ->IC13,9 accent
PI 2 ->9 -> Gate

 

[PaulStoffregen]

Without a schematic, I really can't follow any of this.

But I can tell you CMOS chips (like all CD74ACxx parts) have very high impedance inputs. If nothing is driving the input, and there isn't a pull up or pull down resistor to define the logic level, usually touching the pin or a wire connected to the pin will cause the voltage at the pin to rapidly change as your body picks up electrostatic and magnetic interference from anything nearby. Typically that will make the pin oscillate at 60 Hz or 50 Hz depending on your country's power system, as most indoor EM fields will be at the power line frequency. If your body touches other metal objects, usually you'll see an effect because your skin slightly conducts and the input to the chip is such high impedance (sensitive) that it really doesn't take much to cause it to change voltage which the transistors inside the chip treat as a logic high or logic low.

 

[j303]

Hi Paul. Thanks for that insight, indeed, holding the reset high helped. I get consistent blinking and everything is predictable.