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Real Time Clock Core

Every FPGA project needs to start with a very simple core. Then, working from simplicity, more and more complex cores can be built until an eventual application comes from all the tiny details.

This real time clock began with one such simple core. All of the pieces to this clock are simple. Nothing is inherently complex. However, placing this clock into a larger FPGA structure requires a Wishbone bus, and being able to command and control an FPGA over a wishbone bus is an achievement in and of itself. Further, the clock produces outputs that can be used to strobe an interrupt line. Reading and processing that interrupt line requires a whole 'nother bit of logic and the ability to capture, recognize, and respond to interrupts. Hence, once you get a simple clock working, you have a lot working.

Included in this repository are several basic cores which can be used for this purpose:

• rtcclock: the original RTC Clock module. This was originally built for a Basys-3 board, and so it also has outputs suitable for commanding LEDs and a seven segment display.
• rtclight: Just the basic RTC, with no LEDs or seven segment display output wires.
• rtcgps: A real-time clock which can be used together with a cleaned-up GPS PPS, to keep the clock accurate at a subsecond level to the top of the second. Further work is required to get the clock to the correct second, but this will hold it to the correct subsecond interval.

Since this repository was originally created, the component pieces of the various clocks have been refactored. These are now separate components:

• rtcbare: is the bare bones clock itself. This depends upon a PPS signal being given to it, but other wise it will keep track of the time in a BCD formatted register.
• rtctimer: is a BCD count-down timer. Once zero is reached, an interrupt is created.
• rtcstopwatch: is a BCD stopwatch that generates a BCD stopwatch containing centi-seconds (10ms) resolution, seconds, minutes, and hours.
• rtcalarm: This is a simple component, since it works by comparing the current time against a saved value. Once the value is reached, an alarm is set as an output interrupt wire.