[SOLVED] Is my pull down resistor too small?

Dear community!

I am new to these forums and I'm also new to electronics, it sure is a wonderful hoby. Please excuse me for my grammar mistakes, for english is not my native language. I stumbled upon a problem with my circuit. I have a PIC microcontroller and a parallel (not serial) SRAM in which I succesfuly store data and also read data.

Now, the problem occours only when data is present on all or almost all of the eight SRAM data pins. For example, if the output value is 11, the output is high enough for my pic microcontroller to read a logical high where it indeed is high. But when the output value is 255 (just an example), all eight pins are high, but there is a voltage drop so high, that my microcontroller cannot read a logical high. I really did my best to try fixing the problem, I've been searching all around the web on how to make it work. I tried amplifying signals with an npn transistor, but it does not work. Now I suspect, that my pull down resistors are way too small. I've heard that 10 kOhms are a common practice for pulling down voltage. However, I use only 330 Ohm resistors for a pull down on all eight pins. Am I on the right track towards solution, if I replace this small resistors with 10kOhm resistors? Would that explain my voltage drop (when all eight, or seven, maybe even six pins are high)? But why does it work with 2 or 3 pins being high?

I hope you can help me with that. Be asured of my gratitude. I really want to get this thing to work, but I am also a beginer.:sad:

Have a wonderful day

hi,
Assuming a supply of +5V and a 330R thats a current of 15mA per pin, so for 8 pins thats approx 120mA.

As a PIC is CMOS technology, a 10K pull down would be advisable.
I would ask WHY is a pull down resistor on an Output pin, required on your project.??

E

esp1 said:

hi,
Assuming a supply of +5V and a 330R thats a current of 15mA per pin, so for 8 pins thats approx 120mA.

As a PIC is CMOS technology, a 10K pull down would be advisable.
I would ask WHY is a pull down resistor on an Output pin, required on your project.??

E

Click to expand...

Oh yes, I am sorry for forgetting to list some specifics. It is a +5V yes. Microcontroller is: PIC16F627A. A pull down is required because otherwise I have a floating input on PIC microcontroller and I get weird data.
I am sorry if that does not help you, but I just don't really understand much about pull down resistors. Do you think that replacing my resistors would do the trick?

hi,
I would replace the 330R with at least 4K7 or 10K, this will also help your PIC to run cooler [ 0.12A *5V is 600mWatts!!] and reduce the burden on the power supply.

An Output pin does not really need a pullup or down, its the Input pins which should not be left floating, ie: disconnected.

Are you aware that many PIC's have internal 'weak' pull up resistors, ~50K, which can be enabled/disabled in the program.?

E

Thank you for your reply. I'm going to change the resistors and see how that goes. The thing is that my SRAM chip has 8 bi-directional data lines, so it can be both an input and an output. It depends on control mode. If you are writing or reading from SRAM. I will let you know how that will go. I'll experiment after work.

esp1, thanks for your suggestions. It's working now. First I removed the ressistors, and place 10 kOhm resistors (instead of 330 ohm) to pic input pins. Works great!

I am really grateful.