_"The Cricket!"_ by Street Electronics Corporation, released in 1984, is a hardware peripheral for the Apple //c computer. It plugs into the serial port and offers a multi-voice sound synthesizer, a speech synthesizer, and a real-time clock.
I ended up disassembling both [NS.CLOCK.SYSTEM](../ns.clock/ns.clock.system.s) (to understand the SYSTEM chaining - what a pain!) and The Cricket!'s [PRODOS.MOD](prodos.mod.s) and melding them together, adding in the detection routine following the protocol in the manual.
In the Cricket manual there is a short sample BASIC program to set the clock without the use of any assembly routines, using `PR#2` to talk to the serial card followed by `PRINT` statements. This does not appear to work from ProDOS with the clock driver in place - the time ends up temporarily scrambled until the clock sorts itself out again. Assembly language routines do work, however. I believe ProDOS attempts to read the clock during the execution of the basic program, which interferes the device.