Mesa 7I73+LCD Question

Hello,

I am using a Mesa 7I73 with the following LCD. It is a 4x20, 4bit/8bit interface LCD with backlight and contrast adjust.
C204C-YTY-LW65_Spec.pdf

According to the 7I73 manual typically 8-bit LCDs can be made 4-bit by tying DB0-DB3 low. According to the LCD manual it shows DB4-DB7 being the unused bits rather than DB0-DB3 when in 4-bit mode. I assume that this is just semantics and doesn't matter in the end. I can test it to find out.

My main concern/question is that, other than mentioning DB0-DB3 briefly for this note about 4-bit interfacing, the 7I73 manual appears to offer no further indication of which pins connect to the LCD in which order. The manual does state that LCD0-LCD3 are currently unused, so my best guess would be that my 4 LCD pins will be LCD4, 5, 6, and 7.

Is that correct? If so, does it matter which order they are connected? (LCD4 = DB0 or LCD4 = DB3)?

Thanks in advance.

IMHO the 7i73 manual is right. I've tested many brands of LCD and they always all worked using the LAST 4 address line (DB4 to DB7). I've never seen any working with the first 4 address lines. (most will allow to simply left the first 4 lines unconnected)

Yes, the order is important. LCD4 goes to DB4, LCD5 -> DB5, ... LCD7 to DB7. This will NOT work if address are not in the correct order.

Also, a frequent issue is with the contrast adjustment (VO, pin 3 of the LCD) if it is set wrong nothing will be visible and can make things hard to debug. Do not forget to build a circuit same as the "contrast adjustment circuit' on page 4 (left is right) of the manual. As soon as you power up the lcd you should be able to see 'something' if the contrast is right.

JPL,

Very insightful advice. Thank you. On the LCD manual it appears to show all of the instructions assuming 8-bit operation... I'm sufficiently confused. I've got a Raspberry Pi laying around that I will connect to the LCD this evening to figure out which way it works, but I expect you are right because everything I see online agrees that typically you would use DB4-DB7.

I really, really appreciate your comment about contrast adjustment. I may have gotten it wrong I believe if you hadn't said anything.... I actually have some confusion here also. You said:

"contrast adjustment circuit' on page 4 (left is right) of the manual.

but I was thinking that since I had a VEE pin I would be using the right-hand diagram.

The Mesa manual (page 5) labels LCD11 as VEE and page 4 of the LCD manual shows VEE connected between VSS and V0 (albeit it shows a 10k-20k variable resistor in the diagram). According to the Mesa manual VEE is a 0-3.3V output controlled by a value in an internal register. I was actually thinking that I would connect LCD11 (VEE) directly to V0; my concern was that it looks like a "maximum" contrast would be 5V per the LCD manual which I would not be able to achieve with the VEE pin.

Obviously I need to test the contrast also when I connect it this evening.

If you do use the (LCD11) VEE pin from the 7I73 then you just have to connect it directly to the VO pin of the LCD, nothing else. Note the the VO pin of your LCD was named VEE on the Kyocera C-51847NFQ-LW-AAN, but is is surely doing the very same thing. This _may_ work as mentioned in the manual, but for that the NVCONTRAST value has to be right AND your LCD has to be compatible with a 0-3.3v contrast adjustment...

This being said, the whole idea of using the wiper of a potentiometer between 5V and 0V is to avoid using the VEE pin from the 7i73, thus avoid any problem that may arise if the contrast is not set right in EEPROM.  Which is why I recommended going with the left hand diagram on page 4, (with the pot between 5v and 0v). This will allow full range of contrast adjustment.

Also note that contrast adjustment goes from 'see nothing' to 'see nothing' with a narrow band of 'see something' in between. Too low the screen will be completely dark, too high you will see only bright 'square' with all the pixels lit....  Once you get it right and you know that everything else is working then you can disconnect the contrast adjustment potentiometer and connect the VEE pin (from 7i73) to the LCD VEE  (aka VO) pin and adjust the vcontrast value.

I also see that the confusion is also in part due to the fact that the some LCD manufacturer will name the pin VO or VEE while it is exactly the same thing... (see attached document for the reference design with the kyocera LCD mentioned in the 7i73 manual)