Saturday, April 22, 2017

VFD Driver test board

I decided I didn't want to design, fabricate, and assemble a complete replacement PCB for the Canon P170-DH calculator without testing the various subsystems individually first.

I've already documented the work I did prototyping the printer interface. Although that test used a PIC instead of an FPGA, I have little doubt that I can make that simple bit work with an FPGA with the addition of a couple of Schmitt trigger buffers (the PIC inputs I used have ST inputs).

Next up is the Vacuum Fluorescent Display driver. I did some static testing on my breadboard, but the idea of wiring 12 anode drivers and 13 grid drivers doesn't sound like fun. I decided this would be a good project to develop my KiCad skills.

Here's a screenshot of the board editor window (click to view a larger image):

I'm getting more used to KiCad's UI. The router has some really nice features and some really annoying ones. I won't go into detail here. I should have this board tidied up and sent out by Monday. I'm planning to give Osh Park a try this time, though I have a feeling I'm going to have EasyEDA fab the final 4-layer board just based on price.

I resolved the issue that was driving me to use the Toshiba RN4904. I was trying to fit two drivers into the roughly 6mm spacing of the VFD's pin pairs to keep the drivers aligned with the pins. The SOT-363 is 2.0mm in its shortest dimension which allowed me to squeeze it in just barely, but I wasn't sure how to handle the one spot where there is a group of three pins. And I really don't want to use this tiny package if I don't have to.

Eventually it dawned on me that I was trying to meet an absurd and totally unnecessary requirement. The space on the underside of the calculator's PCB where I'd planned to put these drivers, behind the slide switches, is more than 22mm (0.9") high and over 170mm (6.7") wide. Further, there is a support rib that presses against the back of the PCB between the VFD and that space, so I can't mount the drivers right next to the VFD's pins anyway. The SOT-23-6 package of the RN4604 is 2.9mm wide, so if I mount them with a 4mm pitch they occupy a 100mm (3.9") wide by 12mm (0.47") high area. Oodles of room!

Mounting SOT-23-6 and 0603 packages is no cakewalk, but it's far easier than using SOT-363 and 0402 packages!

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