HP3548 / HP3457 OLED display

[Xyphro]

@kawal: I'll make an FTDI or FX2 playback your recorded pattern streamed from PC, then I can debug faster and see why the display does not take some of the updates.

Update: Streaming pattern generator made (based on FT2232H module + Python). Found then the issue very quickly in why the LCD was not working properly on Kawals HP3852A instrument and could fix it. Will now integrate the rest of the newly spotted HP LCD commands and then update the GitHub repository.

In case I'll get a trace from HP3548, I can now also see quickly too why the 2 issues occur (e.g. shift annuciator not deleted). The recordings can be as long as required, also multiple seconds/minutes.

https://youtu.be/jlces-Pgqd4

[kawal]

FW from XYphro is working excellent in my HP3852A.
 I did have a slight issue with the voltage dropping too much when 2 displays are connected. one display would work no problem but 2 displays would not work together.   There is a 540ohm(270ohm on +5V and 270Ohm on GND) series resistor on the power lines going to the LCD on the HP3852A. I replaced one of them with 100 ohms and its working - probably best to replace both with 100 ohms or even less. Other units have 1K and might be best to lower them as well to get better immunity to the voltage drop. 
Startup

Scrolling across 2 screens

scrolling across 1 screen 2nd screen displaying stationary message

 Kawal

[kawal]

During the testing I managed to measure the current for the 2 displays running off the same power source.

Max current while displaying on 2 displays was 20mA @ 12.5V  - using the switching regulator board version Average was a little less = 13mA.  Max inrush was 112mA when first  applying power. 
Most of the display was lit up as per picture. So overall 10mA per display @12.5V input or 125mW per display .

Kawal

[Xyphro]

Yes, I was afraid of drawing too much current from a supply which was not dimensioned to deliver the additional mAs - that's why I picked a buck converter approach.

Do I understand right that the 2 x 10mA average for you causing a too high voltage drop for your instrument?

Thabks for the logic analyzer trace, which enabled me to test it here and the testing.
There are 6 more commands to be implemented to have them all - other instruments might use them too. In total there are 16 commands to update tje characters, 3 were used by hp3457a, 1 more was missing for your instrument.
Once done I'll update the github and post a message here.

[Xyphro]

Ah, only got it now. You talk about the resistor on instrument side.

HP3547a has R548 on the Main PCB supplying 5V to power the 5V side of the isolator. I had put a note for it on the PCBs back side.
Now for your instrument it's a different situation with 2 Rs it seems

Edit: as Ko4bb.com has an issue and I don't have access to the service manual, do you remember the reference designstoes for those 2 resistors? Would like to put those too on the PCB silkscreen.

[Miti]

Hmmm, I wonder why my display takes more than 60mA, and most of it through the 3.3V line. I get it that I'm using linear regulators, but the power calculation does not add up. I can't see anything wrong with the schematic or the configuration. The schematic around the OLEDs is identical to Xyphro's, the only thing that I can think of in the configuration of the OLEDs is to disable the charge pump, which I did. 

I'll do some more measurements in the weekend to see exactly how much current goes through each supply line.

[wkb]

Not too sure if the idea of switching regulators in a DMM is a good idead, noise-wise I mean?

[kawal]

There are many instruments with switching regulators and that is not an issue.  The analog sections are usually powered separately and isolated from the digital ground. These have multiple MCU's  for sending the data . Even LCD based meters will have converters to drive the LEds or CFLs. 
I know that for the most part you try to avoid noisy power supplies but these are not connected to the analog sections.

I think with such small currents there is no worry for EMI and if there is EMI it would be common mode which the meters excels at removing.

Kawal

[Miti]

Kai,

What did you use to edit the digits .gif file? If I use Paint, any save adds some polka dots on the picture. 

Thanks,
Miti

[kawal]

Ah, only got it now. You talk about the resistor on instrument side.

HP3547a has R548 on the Main PCB supplying 5V to power the 5V side of the isolator. I had put a note for it on the PCBs back side.
Now for your instrument it's a different situation with 2 Rs it seems

Edit: as Ko4bb.com has an issue and I don't have access to the service manual, do you remember the reference designstoes for those 2 resistors? Would like to put those too on the PCB silkscreen.

Yes there are 2 Rs - one is on the +5V rail and one is  on the Ground rail. These are 270 ohm each and the are designated R205 and R206 . I replaced one of them with 100 ohm  from the ground side.  The isolator chip current  is definitely the issue as it is specified between 1mA and 3 mA  quiescent current. Looking at the operating frequency of these displays below 100khz we can assume idle current and working current to be the same.  That would create a  voltage drop on the power rail for the ISO chip..  I guess I got lucky that it was not a problem for the other Meters or Power supplies that I installed the OLED. I think these instruments have 1k resistors.

I would say that because of the operating spec we should keep the resistors  low value - 220 ohms maybe max per display.  That will limit any drop to 0.66V . I am guessing these offer some protection from plugging in the display backwards in the original design so we should probably respect that.  But after deeper dive - if you connect the display cable backwards I see  the +5V will go to a No connect and Ground will go to INA input. So in theory no damage would be done. My instrument also as a keyboard matrix with the same connector type - more likely the resistor is to protect from plugging in the keyboard into the display port . That could destroy the carbon pads on the keyboard - maybe something HP was trying to protect against?  A 220 ohms resistor would allow a short circuit current of 23mA.  A little high for carbon  switch  but will have to be OK.
These might have had dual purpose - one a protection purpose  - 2nd function RC low pass to prevent back feeding noise.

Either way 100 ohms each in HP3852A will drop max 1.2V (0.4,,,1.2V) and seems to work just fine. I will measure tonight the actual voltage drop but chip to chip it a vary. So if you have issue with your display this might be the problem - too high dropper resistors on the power rails.

Kawal

[Xyphro]

Kai,

What did you use to edit the digits .gif file? If I use Paint, any save adds some polka dots on the picture. 

Haha, "Polka dots" that made me laugh hard!  I used GIMP

[Xyphro]

Hmmm, I wonder why my display takes more than 60mA, and most of it through the 3.3V line. I get it that I'm using linear regulators, but the power calculation does not add up

My guess is different scenarios=display content. I doubt your regulators have a lot of quiescent current like the old ld1117's.

Either way there are for both solutions (LDO and DCDC) good and bad effects (potential noise vs. heat/current figures) and it's more a matter of testing for impact or believe.

[kawal]

Hmmm, I wonder why my display takes more than 60mA, and most of it through the 3.3V line. I get it that I'm using linear regulators, but the power calculation does not add up. I can't see anything wrong with the schematic or the configuration. The schematic around the OLEDs is identical to Xyphro's, the only thing that I can think of in the configuration of the OLEDs is to disable the charge pump, which I did. 

I'll do some more measurements in the weekend to see exactly how much current goes through each supply line.

I think you are causing the current to flow because you connected 2 isolated grounds together.  One is going via pin 4 on the display input with 1K protection resistor. The other ground you are just splicing into randomly.  I think this is the issue. These grounds are likely at different potentials. That's why the original design had an isolator. 
These HP devices have many floating grounds.  You could be lucky or not depending where you connect your power ground.


After more careful review of your schematic it seems your design should not work at all.  There is a short between Power and ground.

PWR_ Flag is connected to Ground in your pdf and to +13V at the same time.

Kawal

[Miti]

I think you are causing the current to flow because you connected 2 isolated grounds together.  One is going via pin 4 on the display input with 1K protection resistor. The other ground you are just splicing into randomly.  I think this is the issue. These grounds are likely at different potentials. That's why the original design had an isolator. 
These HP devices have many floating grounds.  You could be lucky or not depending where you connect your power ground.


After more careful review of your schematic it seems your design should not work at all.  There is a short between Power and ground.

PWR_ Flag is connected to Ground in your pdf and to +13V at the same time.

Kawal

Huh? There's only one ground in the digital section, and that is connected to pin 4 of the display connector and to the earth ground. I don't understand where I connect grounds together.
I think I figured it out. I assembled two boards, and the second one is in line with your measurements, about 17-18mA at 13V, about 11mA going through 3.3V, and about 7mA through 8V.
Now I'll have to see where is the difference in current going. Short, bad capacitor, regulator? 

[kawal]

I think you are causing the current to flow because you connected 2 isolated grounds together.  One is going via pin 4 on the display input with 1K protection resistor. The other ground you are just splicing into randomly.  I think this is the issue. These grounds are likely at different potentials. That's why the original design had an isolator. 
These HP devices have many floating grounds.  You could be lucky or not depending where you connect your power ground.


After more careful review of your schematic it seems your design should not work at all.  There is a short between Power and ground.

PWR_ Flag is connected to Ground in your pdf and to +13V at the same time.

Kawal

Huh? There's only one ground in the digital section, and that is connected to pin 4 of the display connector and to the earth ground. I don't understand where I connect grounds together.
I think I figured it out. I assembled two boards, and the second one is in line with your measurements, about 17-18mA at 13V, about 11mA going through 3.3V, and about 7mA through 8V.
Now I'll have to see where is the difference in current going. Short, bad capacitor, regulator? 

Looks like i misunderstood the PWR-Flag - Power flag is not a connection it's a meta flag - clear .  Additionally mixed up the power connections on the 3457A . I got so stuck fixing the HP3852A that i forgot the other units are not the same... The 3457 have a real ground on Pin 4 unlike the 3852A which has a 270 ohm resistor on pin 4.
 
With the isolator design you can take the 13V and GND  from any supply and even take AC . You are not limited to taking power from the same section. I figured you tapped into a section that is isolated from digital.   

[Miti]

Whoops, I touched the OLEDs controller area and one was a bit warm. I resoldered that module with flux and now the current is about 17mA.

Cooool.... literally! 

[Xyphro]

I saw you posted a picture showing 85C. What is the temp now with the fix and zener?

[kawal]

Whoops, I touched the OLEDs controller area and one was a bit warm. I resoldered that module with flux and now the current is about 17mA.

Cooool.... literally! 

Mistakes will happen - we just need to learn form them and not repeat too often. Glad you got it sorted. 
BTW
The zener will help to reduce the localized losses but not overall heat output. The heat output will be the same just speard over more things

[Xyphro]

With 17mA average total power dissipation will be maybe around 220mW.

[Miti]

Mistakes will happen - we just need to learn form them and not repeat too often. Glad you got it sorted. 
BTW
The zener will help to reduce the localized losses but not overall heat output. The heat output will be the same just speard over more things

I removed it, it's not necessary anymore. Both regulators are cool as a cucumber.

[Xyphro]

A cucumber in August timeframe in Tokyo or more Denmark? :-)

Just joking. Very good results!

[Miti]

A cucumber in August timeframe in Tokyo or more Denmark? :-)

Just joking. Very good results!

  Man, if you ever come to Toronto, we must have a beer.

[kawal]

A cucumber in August timeframe in Tokyo or more Denmark? :-)

Just joking. Very good results!

  Man, if you ever come to Toronto, we must have a beer.

Let me know when that meeting is planned  - I want to be invited as well.  Love beer and travel via Toronto once in a while.

Kawal

[Miti]

Let me know when that meeting is planned  - I want to be invited as well.  Love beer and travel via Toronto once in a while.

Kawal

Any time you come by, just send a PM.

[Xyphro]

Canada is one of the few countries I have never been to & I love beer. I'll ping you once it happens :-))

Interim update: All those 16 commands are implemented now and in testing phase!

#define   CMD_WRA12L  (0x0a) /* = 0x0A x4 = 0x028 = write registers A for the 12 digits */
#define   CMD_WRB12L  (0x1a) /* = 0x1A x4 = 0x068 = write registers B for the 12 digits */
#define   CMD_WRC12L  (0x2a) /* = 0x2A x4 = 0x0A8 = write registers C for the 12 digits */
#define   CMD_WRAB6L  (0x3a) /* = 0x3A x4 = 0x0E8 = shift left 6 digits and write registers AB for the 6 digits */
#define   CMD_WRABC4L (0x4a) /* = 0x4A x4 = 0x128 = shift left 4 digits and write registers ABC for the 4 digits */
#define   CMD_WRAB6R  (0x5a) /* = 0x5A x4 = 0x168 = shift right 6 digits and write registers AB for the 6 digits */
#define   CMD_WRABC4R (0x6a) /* = 0x6A x4 = 0x1a8 = shift right 4 digits and write registers ABC for the 4 digits */
#define   CMD_WRA1L   (0x7a) /* = 0x7A x4 = 0x1e8 = shift left 1 digit and write registers A for 1 digit */
#define   CMD_WRB1L   (0x8a) /* = 0x8A x4 = 0x228 = shift left 1 digit and write registers B for 1 digit */
#define   CMD_WRC1L   (0x9a) /* = 0x9A x4 = 0x268 = shift left 1 digit and write registers C for 1 digit */
#define   CMD_WRA1R   (0xaa) /* = 0xAA x4 = 0x2a8 = shift right 1 digit and write registers A for 1 digit */
#define   CMD_WRB1R   (0xba) /* = 0xBA x4 = 0x2e8 = shift right 1 digit and write registers B for 1 digit */
#define   CMD_WRAB1L  (0xca) /* = 0xCA x4 = 0x328 = shift left 1 digit and write registers AB for 1 digit */
#define   CMD_WRAB1R  (0xda) /* = 0xDA x4 = 0x368 = shift right 1 digit and write registers AB for 1 digit */
#define   CMD_WRABC1L (0xea) /* = 0xEA x4 = 0x3A8 = shift left 1 digit and write registers ABC for 1 digit */
#define   CMD_WRABC1R (0xfa) /* = 0xFA x4 = 0x3A8 = shift left 1 digit and write registers ABC for 1 digit */