SDG #08 Symmetricom GPSDO - Noritake VFD Interfacing - Part 2

Part two of the Symmetricom GPSDO project moves from teardown to firmware work, with the focus on getting a Noritake CU20025SCPB 2x20 character vacuum fluorescent display communicating with a Microchip dsPIC30F3013. The toolchain is MPLAB X paired with an MPLAB Real ICE for in circuit debugging, which matters because character display initialisation is one of those tasks where a single mistimed strobe or a missed busy flag check leaves a blank panel with no indication why.

The Noritake module is a useful choice for instrument front panels. It uses a broadly HD44780 compatible command set, so existing character LCD experience transfers, but the VFD gives far better contrast and viewing angle than a typical STN LCD and needs no backlight. Brightness is software controllable, which is worth planning for early if the finished unit will sit on a bench in a dim laboratory. The trade-off is higher current draw and a display technology that ages, so whether the panel will be lit continuously deserves consideration.

Alongside the display work, the GPSDO’s own PCB is examined to find a practical tap point for the OCXO’s 10MHz output. That is the real prize with surplus telecom timing hardware: a disciplined oscillator locked to GPS makes an excellent house frequency reference for calibrating counters, oscilloscopes and signal generators, at a fraction of the cost of a purpose built laboratory standard.

The end goal is a front panel showing time of day and date, driven directly by the microcontroller rather than a pre-built display module, which keeps open the possibility of status indication and menu features later.