186: Sunday 24th April 2011

Started off by rewiring the usb interface as the outputs were for open collector transistor drivers and only 0.6v. There was a 5v output to each of the LEDs, so I soldered a connection to a terminal strip with screw terminals. I used a double sided sticky pad to mount this on top of the chips. I then made up a simple ribbon cable with 9 connections, one for each of the 8 outputs from the usb interface and one for ground. This had a strip of pins on the other end so I could plug it into the prototype board. I then figured out which of the bits I need to change to send a 'display on' and 'all points on' messages. The rest of the bits were wired directly high or low. I know this is really crude, but it is just to get the display up and running. I cleared up the desk with my computer on it and moved my prototype board and the display etc. over there with the usb interface. You can see this in the photo below.
After a thorough read of the data sheets, I found a couple of minor issues, but there was a voltage converter inside the chip that was supposed to generate a large negative voltage supply for the display. This was not working and I had the capacitors the wrong way round at one point as the two data sheet showed different polarities. I trust the Epson chip spec sheet as this is the source document. I could not get the display circuit to produce the voltage so I consulted the notes on how to connect an external voltage. I have a split rail supply on the prototype board as well as a separate 5v supply and the positive and negative rails are separately adjustable. I linked the 5v supply ground and the split rail supply ground so they had the same reference. The input voltage needed to be -9v with respect to the positive 5v supply, net effect is -4v with respect to ground. I set the negative rail to -4v and the display flicked on when I powered up. This was progress. I could see like a grid when I sent the command to switch the display on and then another very slight change when I sent the instruction to switch all points on. When I then sent the command to switch the display off, it went right off. I had to use a wired reset low to get the display back, or switch off the power supply. In the photo below you can see the display with a blue square in it. I tried sending some data and it didn't make any difference, but I had to go for dinner and finish up for the day. I have a feeling there is something else I need to do to select the right brightness level or something as I was expecting all the dots to illuminate with the 'all points on' request. I have made progress as the display is visibly responding to my requests. Step by step I will get there. I did a bit of a rewire and I shall use 2 analogue outputs from the usb interface and the 8 outputs exactly as the eight bits for the data. I only need to control the enable signal and the data or control line (A0) that are now connect to analogue outputs. I shall switch these directly from min to max to go from 0v to 5v, so they are just like the digital signals. If anybody has ideas why I am not getting any dots lit up, I'd appreciate your help.

No comments:

Post a Comment