Electrical – Attiny 45 use reset pin as analog input

The SPI MISO function is nothing but a digital input pin.

The question does not specify whether the pseudo-SPI LED peripheral provides a handshake or other data on the MISO line at all. Assuming there is some initial data at set-up if at all, but no further use of the line by the peripheral during normal operation, the following should work:

Connect one of your 3 data source lines, OR'ed with the MOSI coming from the SPI peripheral, into pin 6 of the ATTiny.

During SPI set-up, ignore (or do what's needed with) the signal coming into pin 6. After that, simply read Pin 6 as you would any normal digital input pin.

If the LED peripheral works fine without connecting it to the MISO on the ATTiny at all, then you can skip the OR connection, and simply treat pin 6 as a standard digital input.

For reference, here's a link to the ATTiny84 datasheet.

Section 16.13 specifies the ADC-related registers for controlling the ADC peripheral.

16.13.1 specifies the ADMUX register:

Bit 7 6 5 4 3 2 1 0
0x07 (0x27) REFS1 REFS0 MUX5 MUX4 MUX3 MUX2 MUX1 MUX0

If REFS1=0 and REFS0=1, the ADC will use the external reference on pin PA0.

These registers can be modified in code, here's an example using AVR-C:

ADMUX = 0x40;

Note that this will mess up the MUX settings, there are other ways to do this if you don't want to mess up the mux settings using bitwise operands:

ADMUX = (ADMUX & 0x7F) | 0x40;