Motorola 68000 Family Primer
With a little creativity, you’ll find dozens of uses for these. For example, one of the synchronous serial port’s modes allows 9-level D/A conversion, which is quite adequate for DTMF generation and even non-critical speech playback. The 6522 does have a bug in one of the serial modes, shifting in under control of an external shift clock, which makes it lose bits if you don’t use a flip-flop so the external clock edges are presented to CB1 only on the rising edge of Φ2, keeping them away from the troublesome falling edge. The bug does not affect most users, and there is a way around it anyway (as shown in the link), but it would be nice if WDC would fix it. I do like WDC’s 65c22 above the other brands because when the pins are input mode, they present a CMOS load instead of LSTTL. This has been very valuable sometimes. The output drivers of the 65c22 (ie, CMOS) from all manufacturers are much stronger than those of the NMOS, despite the overconservative datasheets. In my own experiments, WDC’s can even pull up with 19mA to 4.2V.
The 6551 ACIA has everything you need for a complete RS-232 (TIA-232) link except a crystal and the line drivers and receivers. It includes the onboard baud-rate generator (unlike the 6850 which lacks it, which is why I do not recommend the 6850) and five handshake lines: RTS, CTS, DSR, DTR, and DCD. My RS-232 primer tells about it. You might not think you have any use for an “antique” RS-232 serial port connection, but UARTs (including this ACIA) are used even for interfacing to some modern displays. RS-232 is still used in industrial environments where they need a longer range than you can get with other things like USB which are more oriented toward consumer electronics and are more “black box” and not as hobbyist-friendly. The ACIA can also be used for RS-422 and RS-485 which use different line drivers and receivers but may use the same communications protocol.