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Digital addition

Khazan

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Hello experts:

As the attached schematic shows, I am experimenting a circuit for adding two numbers represented by two digital counters and two cascaded 4 bits adders (74hc283). I tested separately with cascaded 74hc163 and with asynchronous 74hc4040 counters and I ended up with the same questions.

I fee one of the counters (at pin 10) with a 1 khz clk signals and the second counter with 512 hz signals. All signals are well formed square signals.

I use 16hz crystal-controlled time base. The counters count during logic high of the time base that is 1/32 sec or about 31 msec. The counters are reset (at pin 11) in the second half of the logic-low of the time base.

When I verify the outputs of the first counter, my prob realizes pulses at some of the outputs and I consider them as logic high. So that the output of the first counter is binary 00011111 to represent the 1khz clk input, almost as expected. The output of the second counter is binary 00001111 to represent the 512 hz clk input also during the 1/32 sec. So far so good.

I feed input a[] of the cascaded adders by the outputs of the first counter and input b[] of the adders by outputs of the second counter. My expectation is to see the correct sum as binary number 00101110 at output s[] of the 8 bits adder, the same way that I see the correct binary numbers at the outputs of the two counters. But instead, I see binary 00111111 as the sum. If I change the clk input of the first counter to 2khz and I leave the clk input of the second counter as it was 512 hz, I see binary number 01111111 instead of the correct sum 01001110 as sum at outputs s[] of the adder. If I change the clk input of the first counter to 512khz like the second counter, I see the correct output of 00011110 as sum at outputs s[] of the adder.

Is my expectation to see the correct sum at output s[] of the adder justified while the counters are counting during the logic high of the time base?

If so, why do I get wrong sum?

If my expectation is wrong then how am I supposed to get the right sum while the counters are counting during the logic high of the time base?

Please note that it is not about what kind of counters I am using. It is about the wrong sum no matter which counter I use.

Thank you
 

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Khazan

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Hi,

I recommend to be more exact in your naming the signals.
There are some issues, I'm not sure whether you want it this way or not.

Your "283" has inputs A0...A3, B0...B3 like in the datasheet
But it has S1...S4, while the datasheet says S0..S3.
Usually the number is the bit number, so input bit 0 leads to output bit 0. Not in your case.

Usually S0 is the least significant bit.

On the right bottom corner the result is totally confused.
Usually it should be in the order : D7-D6-D5-D4-D3-D2-D1-D0
(while D7 : D4 should be the outputs S3 : S0 of the higher nibble '283')
But in your case output is drawn in the order: D0-D1-D2-D3-D7-D6-D5-D4 (the right most bit (D4) is marked as 'LSB')

Klaus
Hi:
I made a mistake in showing the outputs when I regenerated the schematic and I apologize for that. I attached the correct schematic.
Thank you
 

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FvM

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Without knowing the exact timing of clocks and gate pulses, we can't determine if it's correct to see 15 and 31 or 16 and 32 final counts. We also don't know how the adder result is latched. For the time being I'll assume we see just normal operation. You get out what you put in.
 

Khazan

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Without knowing the exact timing of clocks and gate pulses, we can't determine if it's correct to see 15 and 31 or 16 and 32 final counts. We also don't know how the adder result is latched. For the time being I'll assume we see just normal operation. You get out what you put in.

Hi and thank you for trying to be helpful to me:

I was first testing with 163 counter but I had the same wrong readings at the output of the adder. So I decided to see what happens if I use the 4040. The sum is wrong with both counters.

And I do have a count, pause and reset pulses. I AND gate the clocks with the time base so that when timebase is logic low no clock input is fed to the counters and the counters pause. I also create a pulse (let’s call is pulse-1) right before the reset pulse is generated. So that the two shoulder to shoulder pulses (pulse-1 and reset pulse) of the same length are generate during the low logic of the time base. The pulse-1 provides plenty of time for capturing the output of the adder when using the oscilloscope.

I am not too concerned about the count of 31 and 32 for example as that is solved with synchronous. But I do see variations at t each of the outputs of the adder as a series of pulses at each pin. When I feed the negative of the scope with the reset pulse, I see between 1 and 4 pulses at the outputs. And the sum output is 00111111.
When i feed the negative of the scope with the pulse-1, I see more pulse cycles at each of the output pins because the pulse-1 provides more time to capture outputs. And in this case I see output 01111111.
--- Updated ---

Hi and thank you for trying to be helpful to me:

I was first testing with 163 counter but I had the same wrong readings at the output of the adder. So I decided to see what happens if I use the 4040. The sum is wrong with both counters.

And I do have a count, pause and reset pulses. I AND gate the clocks with the time base so that when timebase is logic low no clock input is fed to the counters and the counters pause. I also create a pulse (let’s call is pulse-1) right before the reset pulse is generated. So that the two shoulder to shoulder pulses (pulse-1 and reset pulse) of the same length are generate during the low logic of the time base. The pulse-1 provides plenty of time for capturing the output of the adder when using the oscilloscope.

I am not too concerned about the count of 31 and 32 for example as that is solved with synchronous. But I do see variations at t each of the outputs of the adder as a series of pulses at each pin. When I feed the negative of the scope with the reset pulse, I see between 1 and 4 pulses at the outputs. And the sum output is 00111111.
When i feed the negative of the scope with the pulse-1, I see more pulse cycles at each of the output pins because the pulse-1 provides more time to capture outputs. And in this case I see output 01111111.
So the 01111111 must be the actual sum produced by the adders. The question that remains is why the sum is wrong?
 

KlausST

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Hi,

This does not give the requested timing information. A textual description is not useful.
****

To further differentiate where the problemmlies: Please confirm:
There is the adder. It has input A and input B and output S.
Tell us all three values. (measured values, not the expected)

Klaus
 

Khazan

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Hi and thank you for trying to be helpful to me:

I was first testing with 163 counter but I had the same wrong readings at the output of the adder. So I decided to see what happens if I use the 4040. The sum is wrong with both counters.

And I do have a count, pause and reset pulses. I AND gate the clocks with the time base so that when timebase is logic low no clock input is fed to the counters and the counters pause. I also create a pulse (let’s call is pulse-1) right before the reset pulse is generated. So that the two shoulder to shoulder pulses (pulse-1 and reset pulse) of the same length are generate during the low logic of the time base. The pulse-1 provides plenty of time for capturing the output of the adder when using the oscilloscope.

I am not too concerned about the count of 31 and 32 for example as that is solved with synchronous. But I do see variations at t each of the outputs of the adder as a series of pulses at each pin. When I feed the negative of the scope with the reset pulse, I see between 1 and 4 pulses at the outputs. And the sum output is 00111111.
When i feed the negative of the scope with the pulse-1, I see more pulse cycles at each of the output pins because the pulse-1 provides more time to capture outputs. And in this case I see output 01111111.
--- Updated ---


So the 01111111 must be the actual sum produced by the adders. The question that remains is why the sum is wrong?
I just realized that when I verified the outputs during the entire pause time (the time that the low time base last) instead of just during the pulse-1 time, the actual output is all ones as 11111111.
 
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wwfeldman

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give one counter one clock pulse - NOT a string of 512 Hz, not a string of 1024 Hz - just one clock pulse to one counter
then measure all of the A inputs to the adder - report it as a binary number
then measure all of the B inputs to the adder - report it as a binary number
then measure all of the S outputs of the adder - report is as a binary number

then give that same counter about 75 pulses and give the other counter about 100 pulses
repeat the measurements and report the results

then give that same counter another 75 pulses and give the other counter another 100 pulses
repeat the measurements and report the results

the goal here is to ensure the correct wiring and operation of the adder

once that's verified, the timing questions will need to be addressed

how are you determining the binary numbers you are reporting?
 

Khazan

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give one counter one clock pulse - NOT a string of 512 Hz, not a string of 1024 Hz - just one clock pulse to one counter
then measure all of the A inputs to the adder - report it as a binary number
then measure all of the B inputs to the adder - report it as a binary number
then measure all of the S outputs of the adder - report is as a binary number

then give that same counter about 75 pulses and give the other counter about 100 pulses
repeat the measurements and report the results

then give that same counter another 75 pulses and give the other counter another 100 pulses
repeat the measurements and report the results

the goal here is to ensure the correct wiring and operation of the adder

once that's verified, the timing questions will need to be addressed

how are you determining the binary numbers you are reporting?
I will try the manual clocking to see the results.
“how are you determining the binary numbers you are reporting?”
As I mentioned in my previous reply, I am using the scope in different ways to read the outputs of the adder. I am confident that I am capturing the right outputs with the scope and my final conclusion was that the sum at the output of the adders is 11111111 Not the number that I initially mentioned in my post.
thank you
 

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Hi,

I recommend to be more exact in your naming the signals.
There are some issues, I'm not sure whether you want it this way or not.

Your "283" has inputs A0...A3, B0...B3 like in the datasheet
But it has S1...S4, while the datasheet says S0..S3.
Usually the number is the bit number, so input bit 0 leads to output bit 0. Not in your case.

Usually S0 is the least significant bit.

On the right bottom corner the result is totally confused.
Usually it should be in the order : D7-D6-D5-D4-D3-D2-D1-D0
(while D7 : D4 should be the outputs S3 : S0 of the higher nibble '283')
But in your case output is drawn in the order: D0-D1-D2-D3-D7-D6-D5-D4 (the right most bit (D4) is marked as 'LSB')

Klaus
The 74HC283 has pins numbered A4-A1 and B4-B1 with the output on S4-S1, so the output is marked correctly and the inputs are incorrectly marked.

It would be better if you showed us a real schematic that has the pin numbers along with the pin names, but I suspect you don't have a real schematic just a Visio (or similar) drawing.

I think you should just report the logic levels on the pins of each part.

In order of most likely issue to least likely issue
1. Wiring problem (pin number/pin name mismatch, incorrect handling of ci/co, etc)
2. Timing problem (glitches in the clock generation from gating, not enough settling time)
3. Measurement errors (may be caused by incorrectly drawn schematic)
4. Part failures (then again if these parts are old and have been handled a lot with no ESD precautions then this might be #1)
 

Khazan

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The 74HC283 has pins numbered A4-A1 and B4-B1 with the output on S4-S1, so the output is marked correctly and the inputs are incorrectly marked.

It would be better if you showed us a real schematic that has the pin numbers along with the pin names, but I suspect you don't have a real schematic just a Visio (or similar) drawing.

I think you should just report the logic levels on the pins of each part.

In order of most likely issue to least likely issue
1. Wiring problem (pin number/pin name mismatch, incorrect handling of ci/co, etc)
2. Timing problem (glitches in the clock generation from gating, not enough settling time)
3. Measurement errors (may be caused by incorrectly drawn schematic)
4. Part failures (then again if these parts are old and have been handled a lot with no ESD precautions then this might be #1)
Thank you for your analysis of the problem.

About the pin numbering, I accept your suggestions. Also, I had concluded that the breadboard (BB-32623 from mouser.ca) which I am using is not reliable. For example, intermittently, the time base generator stops working or produces not well-formed signals. However, after I wiggle the capacitor and resistor pins in the section where the time base generator is set up, the generator works again.

All ICs are new and I had duplicates with which I had already tried to troubleshoot the circuit but the ICs seem to be OK.

I have other components in the circuit and I isolated the two counters and the adder for testing their outputs and I asked questions about my problem in this forum. It is now going to be a pain to dismantle everything and to replace the breadboard but I don’t seem to have a choice.

Another thing that is perhaps unusual is that when the counters and the adder function together, my power supply shows fast variations in the current and power consumption of the circuit. (1) Should I consider that as normal?
 
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KlausST

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Hi,

a schematic is a good information what you want to do.
But a photo is good information how it really is.

Klaus
 

Khazan

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Hi,

a schematic is a good information what you want to do.
But a photo is good information how it really is.

Klaus
I am already in process of disconnection everything. Perhaps this will continue in the future.
Thank you again.
 

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Another thing that is perhaps unusual is that when the counters and the adder function together, my power supply shows fast variations in the current and power consumption of the circuit. (1) Should I consider that as normal?
Did you use bypass capacitors (from VCC to GND) on the supply pins of all the ICs? If you don't have enough bypass you can see issues with current and voltage fluctuations on your power supply. The bypass capacitors are used to supply enough transient current when the ICs switch. From your description you probably don't have enough (or any) bypass.
 

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