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Help: Wonky 0603 Pull-Up Resistor on a board I built


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#1 FarmerPotato OFFLINE  

FarmerPotato

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Posted Mon Sep 10, 2018 10:30 AM

 
I don't have extensive experience debugging hardware, so I'm asking for help with this little problem in my first surface mount build.
 
In this circuit, as built by me, the pull-up resistor R1 to RDBENA* is not accomplishing its job.
 
The LVC245A starts driving when VCC is applied. It's connected to the 4A sideport data bus, so disaster ensues. When I drive RDBENA* to 0 or 1 along with DBDIR, the board works ok.
 
I'm wondering how to explain the measurements before I start over and reflow the parts.
 
U1:
VCC --- C1 --- GND
DBDIR --- BDBDIR
CE* --- R1 --- VCC
    \-- RDBENA*
 
Spec Values:
U1  LVC245A (SOIC 0.05" spacing)  
C1 0.1 uF smd 0603
R1 3.3K smd 0603
I hate 0603s. I'm switching to 1206.
 
Measured values:
C1 0.099 uF
R1 9.9K
R2 3.3K (elsewhere in another pull-up)
 
Voltage measurements with RDBENA driver set to hi-Z:
 
VCC 3.3V
R1  1.6V measured at left side of R1
RDBENA* 1.6V measured off-board
CE* 1.5V measured at chip leg
 
So the pull-up resistor R1 is not accomplishing its job.
Is it a bad solder joint? I verfied continuity between each lead.
The measured resistance of 10K is strange - my R2 was 3.3K from the same reel.
 
 
 
Ideas:
1. Is RDBENA* 1.6V consistent with it being an unconnected, floating input?
 
2. Is R1 damaged by my sloppy soldering, making it 10K?
 
3. Is there not enough current to pull CE* high? From the LVC245A spec, it only needs <0.01 mA. There should still be 0.33 mA available into CE* 
 
1.7V drop over R1 would be 0.17mA flowing. This doesn't make sense. 
 
4. Could the 9.9K measured at R1 instead be the resistance from VCC through U1 to CE*?
 
5. My VCC traces are the same size as every other signal trace. Should they be bigger? (I do have a ground plane fill.)
 
6. I'm leaning toward the theory that left lead of R1 is not connected... except my continuity check tells me it is. Maybe I'm probing the board trace, not the resistor?  
 
7. Suppose R1 is completely destroyed. How much heat would it take to destroy an 0603 resistor?

Edited by FarmerPotato, Mon Sep 10, 2018 10:34 AM.


#2 Stuart OFFLINE  

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Posted Mon Sep 10, 2018 2:06 PM

The LVC245A - isn't that rated for operating up to 3.6V? Will it reliably drive the 5V TTL TI-99 bus?

 

Have you got a circuit diagram? I don't understand where the pull-up resistor is. If its on the RDBENA* output from the card, then I'm not sure it is needed as there's a pull-up resistor in the flex cable circuit.



#3 FarmerPotato OFFLINE  

FarmerPotato

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Posted Mon Sep 10, 2018 2:58 PM

The LVC245A - isn't that rated for operating up to 3.6V? Will it reliably drive the 5V TTL TI-99 bus?

 

Have you got a circuit diagram? I don't understand where the pull-up resistor is. If its on the RDBENA* output from the card, then I'm not sure it is needed as there's a pull-up resistor in the flex cable circuit.

 

It's attached to the side port. 

 

I am using the LVC245A as a level translator. It tolerates 5V inputs, and as for output, the VOH is 3.3V   and is sufficient to pull TTL high.



#4 FarmerPotato OFFLINE  

FarmerPotato

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Posted Tue Sep 11, 2018 9:35 AM

Update:
 
I replaced R1 with a 0805 3.3k. Tested all the continuity. There is still a voltage drop across pull-up resistor R1. It supplies 2.4V to CE* on the LVC245A (still not 3.3V) when nothing else is driving CE*. This might be adequate.
 
I suspect the LVC245A might be damaged and sinking 3ma into CE. I measured total current at the power supply at 10mA, which seems large. 
 
 
Stuart, I messaged you a link to a schematic.

Edited by FarmerPotato, Tue Sep 11, 2018 9:45 AM.


#5 jedimatt42 OFFLINE  

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Posted Tue Sep 11, 2018 11:52 PM

RDBENA* is for PEB cards to control the 74LS245 in the Flex cable boot... To my understanding anything you connect should be high-impedence, or active-low. You should never drive it to 5V, or any volts... or you add more current that other cards have to sink.

If you are on the sideport, you wouldn't interact with it at all... The first version of my external 32k board was simply Theiry's circuit minus the RDBENA control. http://www.unige.ch/...ti99/mem32k.htm


But, where I do mess with RDBENA* on TIPI, in the PEB, I use half of a 74HCT244 to drive RDBENA active low or leave high-impedence, with a pull down resister on one of the inputs, RDBENA connected to the matching output, and then my control logic, such as what I feed the on board '245's enable to the '244s output-enable, either driving active low, or not driving at all.

-M@

#6 FarmerPotato OFFLINE  

FarmerPotato

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Posted Wed Sep 12, 2018 10:03 AM

RDBENA* is for PEB cards to control the 74LS245 in the Flex cable boot... To my understanding anything you connect should be high-impedence, or active-low. You should never drive it to 5V, or any volts... or you add more current that other cards have to sink.

If you are on the sideport, you wouldn't interact with it at all... The first version of my external 32k board was simply Theiry's circuit minus the RDBENA control. http://www.unige.ch/...ti99/mem32k.htm


But, where I do mess with RDBENA* on TIPI, in the PEB, I use half of a 74HCT244 to drive RDBENA active low or leave high-impedence, with a pull down resister on one of the inputs, RDBENA connected to the matching output, and then my control logic, such as what I feed the on board '245's enable to the '244s output-enable, either driving active low, or not driving at all.

-M@

 

I should clarify, my RDBENA* is internal to my side port thingy, not connected to a Pbox or anything else.

I named it because I think it duplicates the same function as the RDBENA* in the flex hose. It turns on my 245 which connects to the databus at the sideport.

 

If I were making a PBOX card I think I would just call it OE on the board.

 

 

My problem was "fixed" by replacing R1 and C1.

 

I replaced the 0603 pull-up resistor that read 10k with an 0805  3.3k.  There is still a 0.7V drop across R1, which makes 2.5V into CE* suffficient. 1.6V was not!

I think I may have a slightly damaged LVC245A sucking too much current, but today it passes tests.

 

I don't observe any voltage drop across the same circuit , pull up resistor R2, connected to the CE* on another LVC245A.

 

 

BTW, I didn't see any reason to use 244s, I think the 245A pinout simplifies my board when used in one direction only.

 

Hopefully getting closer to a public announcement of a working board...

 

Matt, I am definitely taking your suggestion of daisy chaining onto your 32k sidecar pins. It adds so much value to coexist with a TIPI.

I have your 32k sidecar to begin with, and I'll order a TIPI when I actually have time to use it.

 

-Erik






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