I have some TPIC6A596NE shift registers. I am familiar with connecting and using 74HC595 shift registers, and these should connect and operate the same way, but they are SINKS and handle higher currents. I am wondering about the PGND pins. I am planning on running everything off of a single 5V power supply. So the MCU, these shift registers, and the motors these will drive, will all share a common +5V and GND. I know to connect the LGND (Logic GND) of the shift regsiters to GND, but the PGNDs are unclear. The datasheet says they are internally connected, but then it says that EACH PGND pin must be externally connected to the power system ground. So does that mean I have to externally connect every one of those 4 PGND pins to GND? Or just one since they are internally connected? I will probably connect all 4 to each other, and then to GND to be safe. But I thought I would ask in case somebody can help me understand how this all works. Thanks!
The multiple ground pins are common when a high current is anticipated. In a circuit board they are normally soldered to an extended ground plane or pad which helps with heat dissipation.
I would be more concerned with the common 5v logic and motor supply. If you have to do this then make sure your supply has ample headroom to avoid brownouts. An additional electrolytic capacitor if often helpful.
See page 11 of the datasheet - connect all to Gnd, not just to each other.
I don't know why you couldn't keep this multi-motor drive discussion all in 1 thread.
Make sure you put decoupling caps on all the shift registers and the '328.