[SOLVED] connecting Single output to multiple output

[aminpix]

I am using a stm32 microcontroller to control 20 × 8 (160) pins. I decide to use 8 bit D-Flip-Flop. In my design, each output pin of the stm32 is connected to 20 input the D-Flip-Flop.
Does it cause any problem? Any thing I should concern in my design?

 

[c_mitra]

There is a fan-out spec on every output pin; perhaps each output may not be able to drive 20 input pins. Please refer to the datasheet.

You should better use a buffer on each of the 8 pins.

 

[cariban]

That depends on the speed of the signal. Each input port has parasitic capacitance. Too big capacitance will ruin the high signal since the driver can not reach the expected level in time. Just read the datasheet of the D-gate and find out the maximum port capacitance. Multiply the number by 20 you will get the total capacitance. Then check the output port's driving strength, usually specified by maximum source/sink current. This will help you to estimate how much time the signal will increase/decrease to the
threshold voltage (HIGH 70%, LOW 30% VDD for CMOS). Then you will know if the signal speed is too high or not.

 

[KlausST]

Hi,

additionally you need to consider wiring capacitance.
Wire length plays a role when considering reflections.

It makes a difference if the 20 DFF ICs are all on one PCB with short wiring or independent PCBs with long wiring. Then additionally it plays a role if you use star wiring or straight bus wiring

You see: It can be easy when frequency is low and wiring is short .... and it may become challenging with high speed signals and long wiring.
We don´t know your application. In doubt you need to tell us.

Klaus

 

[aminpix]

Thanks everyone. I decided to use two fanout buffers (with 10 outputs) I think it will be more safe (74fct3807).