On the 68HC11F1, there are two lines for the ADC's internal voltage reference: Vrh and Vrl (pins 67 / 68).

Since my application will NOT be using the ADC at all, what do I do with these lines?

The datasheet just says what the pins are - I don't see anything saying how they should be connected?

Tie them to ground or just let them float?

 

M68HC11 Reference Manual

page 65

Since the VREFL and VREFH pins do not connect to the inputs of any CMOS gates within the MC68HC11A8, these pins do not need terminating if they are not used. Although termination is not required, it may reduce the risk of damage due to high-voltage static electricity.

 

Thanks for that PDF. I only have the manual for the 68HC11F1 version.

I'd tied both together and then to ground, but had found a schematic where Vrl was tied to analog ground (and which had no clear path back to the main ground). Vrh was tied to a circuit that was used to set the reference at a given voltage (but that portion of the board was then left unpopulated).

I've changed it to separate 10K pull-downs on both pins.