PCB design help and suggestions for a beginner

Hello All,

I recently graduated with an electronics degree... I am volunteering at a pcb design companys where my role is engineering, more design and sschematic..... To get started and learning, I am doing a small project to get familiar...... Here is the design basic project I came up with.


1) A clock fan out

2) power supplies for the entire circuit...


Here is my idea:
1) Design a power supply /circuit that converts 5 v to a lower voltage


Here is my question
1) To change the voltage what is the best method to do it? Is a voltage divider effective?
2) What considerations should I keep for a clock circuit?

Any help will be really appreciated....

Try a voltage divider. Find the max and min voltages in and out, and the max and min currents that will be drawn. Calculate the power in the divider elements. Is this practical?
You will probably need to use a regulator chip. Look at the web sites of a few IC manufacturers like National Semiconductor, or Linear Technology. They have web tools to suggest chips to use.

You cannot use a resistive divider to create a low voltage for a power supply. Use a regulator. Be careful of the dropout voltage. I would suggest a linear regulator to keep it simple to begin with.

Keith

SherpaDoug said:

Try a voltage divider. Find the max and min voltages in and out, and the max and min currents that will be drawn. Calculate the power in the divider elements. Is this practical?
You will probably need to use a regulator chip. Look at the web sites of a few IC manufacturers like National Semiconductor, or Linear Technology. They have web tools to suggest chips to use.

Click to expand...

Its for learning purposes with the most practical design in mind. Voltage divider would be simple and small but like you and keith1200rs suggested, a regulator might be more appropriate considering it is going to drive other chips and clocks.

I think LM3940 should be suitable for this application.

Alright, I am stuck again......

So this is how I planned to make the board...

Crystal oscillator to 1 to 4 clock buffer...

Power supply using regulator to distribute power to osc, buffer and gates and other logic devices.....

here is my question
1) For logic gates, what is the best way to provide input? I was thinking about giving supply directly OR using the clocks.... So in case I use clock, is there a way I can change their fcy? eg... if i make the fcy half, then at i can have clocks going 0 0 1 1 and 0 1 0 1 to 2 inputs. how do i make this at board level?

Incase I provide the voltage supply itself, how connecting other input to gnd will provide logic 0? Is there another alternate for me to switch inputs with a push of a button?

2) I want to add test points... would i need resistors?

I am afraid question #1 makes no sense. The logic gate inputs depend on what you want for outputs. If you are using CMOS then connecting an input to the supply is fine. To divide a frequency you need a counter or sequencer. A flip flop is a trivial counter that divides by 2.
#2 if the lines to the test points are short you don't need resistors. For very high frequency work the test probes may have resistors, but not the PCB.

Tell us more of what you are trying to do and we can be more helpful.

SherpaDoug said:

I am afraid question #1 makes no sense. The logic gate inputs depend on what you want for outputs. If you are using CMOS then connecting an input to the supply is fine. To divide a frequency you need a counter or sequencer. A flip flop is a trivial counter that divides by 2.
#2 if the lines to the test points are short you don't need resistors. For very high frequency work the test probes may have resistors, but not the PCB.

Tell us more of what you are trying to do and we can be more helpful.

Click to expand...

Ok, lets forget about the frequency part, using voltage inputs is just fine. So suppose I am using a chip with 3.3 as logic HI and 2.5 as logic LOW.

I am getting these through a linear regulator.....

I want the input to the gate to switch between these 2 points, this is what I planned

O O O
2.5V 5 V / ? 3.3V


So i connect a jumper like this


O--------O O
2.5V 5 V 3.3V
say this will give me 2.5 V... this is method correct? and will using resistors to gnd or vcc make the design better?

Thanks again SherpaDoug. I will write a detailed document explaining the process using schematic tools once its done so this thread can be useful for future users.

3.3V for a high and 2.5V for a low...That is like no logic family I have ever dealt with. Can you give me a actual chip part number or a link to a data sheet? I think you have something wrong here.

Very confusing explanation.. the only standard that i remember now that follow this spec is 3.3V CML.. but that differential.. and probably not the case...

@SherpaDoug and @Sink0

I guess you are right, I think either I misunderstood, or I didnt get the right information. Either ways, whats confirmed is that I need 2 voltages, 3.3 and 2.5V.... And like you both mentioned, there is no gates that have any kind of logic like that.

I guess generating a 0.8V for CMOS logic would also be difficult, so using ground is the most effective solution. Anyhow, to make use of the 2.5V i am getting out of the regulators designed, I decided on using an inverter at one end of the jumper switch, which would manually allow me to switch between 3.3V and 2.5V.

so the jumper would be like this:
|||||||O O O
3.3V 0V 2.5V
Thus, connecting a jumper between 1st and 2nd pin would give be 3.3 and hence a logic 1... Now, since I need a logic 0 (around 0.8V ), I would use an inverter connected to the other end from 2.5V thereby giving me a logic 0....

Now can I use a jumper setting to give me 0.8V (3.3V or 2.5V)? or can anyone suggest something better?

I think you need to tell us more about this really weird application. Either that or give us a link to a datasheet of the logic family you want to use.

Lacking that, if you are sure that +3.3V is a logic 1, then send that logic 1 through an inverter and whatever emerges from the inverter must be a logic 0. Apply those logic signals to the ends of your switch and you will be able to switch between 1 and 0.

I am or was going by the specifics I got, all i got was what I discussed, and this isnt a practical circuit, its just to get familiar and start learning the flow of how things happen in PCB design, so I can use any chip I want and thus no data sheet to refer to... The plan is to make a circuit with a 3 output crystal oscillator, test logic gates with different logics with mentioned voltages and power supplies for the same.


Anyways,

I am going to be using a chip that uses 3.3V and / or as logic 1 and 0 for logic 0. As of now, i am building a circuit this way....

1)Main Power input = 5V

2)Using linear regulators to get 3.3V and 2.5V, both for logic 1 to various gates to test...

3)A 3 pin connector to switch between the logic using a jumper

4) A 1:4 crystal oscillator

5) Test points for checking each voltage w.r.t ground....


I think I am on the right track with the circuit in design now, with the forum and I could clear out a lot of doubts...... Please let me know if I am off anywhere and thanks again for all the help