7 segment display Pin outs?

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Kaellis991
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Re: 7 segment display Pin outs?

#171 Post by Kaellis991 »

Thanks for the input. I really don't have the knowledge to design a circuit but I think I can make one in EasyEDA, if I know what components are needed.
I would like to just duplicate the module board but rearrange the components for my display.

Edit; the diagram I posted was a circuit I found that someone else created for EasyEDA

SimPassion
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Re: 7 segment display Pin outs?

#172 Post by SimPassion »

Please just wait others answer and confirmation before creating this circuit, in order to start in the proper direction ...

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Keith Baxter
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Re: 7 segment display Pin outs?

#173 Post by Keith Baxter »

SimPassion wrote: Wed Dec 28, 2022 1:58 am Please just wait others answer and confirmation before creating this circuit, in order to start in the proper direction ...
Hi.

This is how my com radio is wired. i shared the schematic here. Yes R1 is connected to iset but should NOT also be connect to seg G

https://www.forums.siminnovations.com/v ... 720#p50720

Keith

EDIT: Here is just one max using 2 x 3 digit displays.
Schematic_MAX 6 digit_2022-12-28.png
Last edited by Keith Baxter on Wed Dec 28, 2022 9:50 am, edited 1 time in total.
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jph
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Re: 7 segment display Pin outs?

#174 Post by jph »

SimPassion wrote: Wed Dec 28, 2022 1:58 am Please just wait others answer and confirmation before creating this circuit, in order to start in the proper direction ...
Very sound advice Gilles.

On the schematic Kirk posted you are indeed absolutely correct in that the peak current regulator resistor (RSet) / R1)should only be connected to VCC and ISet pin is shown incorrectly. (it is not 'reset! - there is no 'reset')
A better example (with correct ISet wiring AND resistor RSet (this is resistor R1) is as follows for 8 displays.
kirk7219.png
A note on Resistor R1 - Rset
Connect to VDD through a resistor (RSET) to set the peak segment current (Refer to Selecting
RSET Resistor and Using External Drivers section).
Basically, because this is a multiplexed circuit the peak current is quite high (as it is short duration) 10k is standard giving a peak of around 40mA per segment but leading to an average of around 10mA or so.

40k for RSet (R1) as shown in another example in this thread and also mentions 'reset' ??? This is incorrect and should NOT be used. (I note this has now been edited since my post and the reference removed. That is good as google does not provide all knowledge :lol: )


Otherwise the modified diagram I have shown here now appears correct.

The electrolytic capacitor can be anywhere around 10uF and at least 7.5V. The .1uF can can be any crappy old ceramic. Mount them as close to the power pins on the 7219 as possible.

You only then need to decide as to the order of the digits for use in AM which should be logical and sequential. I would personally follow exactly the same order as AM uses for 8 digits, just with the 2 least significant digits removed. I can explain more if required.

As for info to give to a pcb maker ?, I have no idea as I simply do not bother. More trouble than it is worth unless you are making hundreds of them.
But all looking good.
Joe
Joe. CISSP, MSc.

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Keith Baxter
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Re: 7 segment display Pin outs?

#175 Post by Keith Baxter »

Hi,

Joe is correct. There is no reset. That was a auto correct spelling thing, my bad. All diagrams indicate iset though.

Joe is also correct that a 10K resistor should be used when using 8 digits. However 10K is to low for less digits. 40K is recommended for 4 digits. I use 40K on all my radios.
The correct way is to work out the necessary segment current and forward voltage based on the number of displays.

The basics for the MAX7219 you can find here.

https://www.elprocus.com/max7219-ic/

Keith
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jph
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Re: 7 segment display Pin outs?

#176 Post by jph »

Rather than using odd / obscure websites, just simply use the datasheet (The bible)
https://www.analog.com/media/en/technic ... ax7221.pdf
Hi Keith, you are mostly spot on
Just for a point or refence Your schematics do not show '4' digits ?

The 'bible' says -
Intensity Control
and Interdigit Blanking
The MAX7219/MAX7221 allow display brightness to be
controlled with an external resistor (RSET) connected
between V+ and ISET. The peak current sourced from
the segment drivers is nominally 100 times the current
entering ISET. This resistor can either be fixed or variable
to allow brightness adjustment from the front panel. Its
minimum value should be 9.53kΩ, which typically sets the
segment current at 40mA. Display brightness can also be
controlled digitally by using the intensity register.
Digital control of display brightness is provided by an internal
pulse-width modulator, which is controlled by the lower
nibble of the intensity register. The modulator scales the
average segment current in 16 steps from a maximum of
31/32 down to 1/32 of the peak current set by RSET (15/16
to 1/16 on MAX7221). Table 7 lists the intensity register
format. The minimum interdigit blanking time is set to 1/32
of a cycle.
Great input though, thanks, Joe

Joe
Joe. CISSP, MSc.

Kaellis991
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Re: 7 segment display Pin outs?

#177 Post by Kaellis991 »

Those are all good responses to my question, but what I got out of this is that I need to stick with my original plan.

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jph
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Re: 7 segment display Pin outs?

#178 Post by jph »

Kaellis991 wrote: Wed Dec 28, 2022 11:43 am Those are all good responses to my question, but what I got out of this is that I need to stick with my original plan.
I thought you had a working mode that Keith had worked on - (don't underestimate the time taken for this step)
if that is technically the best or the worst model ? - who cares - providing it works >
Stick with what you have it does the job.
Technicalities' are not the same as results. If you have results, stick with the results.

Joe
Joe. CISSP, MSc.

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jph
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Re: 7 segment display Pin outs?

#179 Post by jph »

Keith Baxter wrote: Wed Dec 28, 2022 10:16 am Hi,


Joe is also correct that a 10K resistor should be used when using 8 digits. However 10K is to low for less digits. 40K is recommended for 4 digits. I use 40K on all my radios.
The correct way is to work out the necessary segment current and forward voltage based on the number of displays.

Keith
Hi Keith, let me correct you on that please.
40K is ONLY recommended for low digit values (low number of digits) IF you also limit the number of digits in the 'scan limit' register.
Since you have no control over the scan limit register in AM, then you are always using 8 digits no matter what you do. The fact that you might only decide to use 3 digits or 4 is of no consequence as the display driver multiplexor is is still refreshing at the exactly the same as 8 digits. Hence you need exactly the same RSet resistor for how ever many digits you use in AM.
40k will of course work, but will give you a much dimmer display.

The theory is quite straight forward. The mux runs a scan of all 8 digits, this takes time. The 'scan' frequency for the digits is set at more or less 800hz .

At 800hz with 8 digits the PEAK current to a digit / segment may be 'high' but it is for a very very brief amount of time. It is the AVERAGE current that you end up 'seeing' and this controls the 'brightness' to the viewer.
There is a table in the datasheet and also a simple formula -
Intensity Control
and Interdigit Blanking

The MAX7219/MAX7221 allow display brightness to be
controlled with an external resistor (RSET) connected
between V+ and ISET. The peak current sourced from
the segment drivers is nominally 100 times the current
entering ISET. This resistor can either be fixed or variable
to allow brightness adjustment from the front panel. Its
minimum value should be 9.53kΩ, which typically sets the
segment current at 40mA. Display brightness can also be
controlled digitally by using the intensity register.
Digital control of display brightness is provided by an internal
pulse-width modulator, which is controlled by the lower
nibble of the intensity register. The modulator scales the
average segment current in 16 steps from a maximum of
31/32 down to 1/32 of the peak current set by RSET (15/16
to 1/16 on MAX7221). Table 7 lists the intensity register
format. The minimum interdigit blanking time is set to 1/32
of a cycle

Scan-Limit Register
The scan-limit register sets how many digits are displayed,
from 1 to 8. They are displayed in a multiplexed manner
with a typical display scan rate of 800Hz with 8 digits
displayed. If fewer digits are displayed, the scan rate is
8fOSC/N, where N is the number of digits scanned. Since
the number of scanned digits affects the display brightness, the scan-limit register should not be used to blank
portions of the display (such as leading zero)
If the scan-limit register is set for three digits or less,
individual digit drivers will dissipate excessive amounts of
power. Consequently, the value of the RSET resistor must
be adjusted according to the number of digits displayed,
to limit individual digit driver power dissipation. Table 9
lists the number of digits displayed and the corresponding
maximum recommended segment current when the digit
drivers are used.
As I said at the beginning, you have ABSOLTUTELY NO CONTROL over the scan limit register (IN AM) so you are always using an 8 digits SCAN MULTIPLEX and RSet should be set for 8 digits ALL the time. - even if you only used 1 digit to display. The SCAN LIMIT register is FIXED at 8 hence the refresh rate of the multiplexed digits are CONSTANT and Rset formula does not and never will apply.
You will NEVER do this as it is not possible for you to do this using AM. It is ALWAYS an 8 digit multiplexer and due to a fixed F0sc of 800Hz the peak current / average current is identical (PER DIGIT) no matter how many digits you use.

Hope that helps. It is all in the datasheet -
https://www.analog.com/media/en/technic ... ax7221.pdf

Joe
Joe. CISSP, MSc.

Kaellis991
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Re: 7 segment display Pin outs?

#180 Post by Kaellis991 »

My goal was to gain some knowledge in creating a simple little PCB and thought that duplicating this module for the two (3) digit displays would be a good learning exercise.
If I had a PCB with all the proper through holes and traces on it I could then solder in the components, all of which I have as through hole devices.

The castellated 7-segment displays could be attached to a separate board with male machine pin headers that would plug into the female pin headers on the primary board.
It seems possible to me, but perhaps because of my inexperience there is something I am missing.

Of course the new board wouldnt have the same dimensions or layout, because the max7219CNG IC I have is a TH type and it won't fit between the machine pins like on this module.

Edit: I will try my hand at creating a breadboard prototype of the circuit that Joe revised in the post above using the capacitors and resistor in the image below. I think the capacitors are the correct ones to use. The resistor is a 10k.


IMG_6094.jpg
|
IMG_6098.jpg

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