Will I need external timing with attiny85?
Will I need external timing with attiny1634?
No, but it will be easier with external timing.
To operate reliably, the transmitter and receiver need to be using the same bit period within +/- 2% (see http://pdfserv.maximintegrated.com/en/an/AN2141.pdf
). Your master is an Arduino, so assuming it uses a ceramic resonator (e.g. Uno) then its clock frequency tolerance will be +/- 0.5%. This means that your ATtinys must have a clock frequency that is accurate to within +/- 1.5%.
The easiest way to achieve this is to use a 3-terminal ceramic resonator on each ATtiny. However, this uses up 2 pins.
The other way is to use the internal clock in the ATtinys, but calibrate it using the OSCCAL register. The clock accuracy of the ATtiny as-shipped is guaranteed to be no worse than 10% @ 3V. You will need to use a regulated supply to feed the ATtinys, because the clock frequency is voltage-dependent. Also, the temperature had better not fluctuate too much, because the clock frequency varies with temperature.
An alternative is to use a self-clocking protocol instead of standard async protocol. if each transmission starts with several alternating 0s and 1s then the receiver can measure the bit length with respect to its internal clock, removing the need for an accurate clock frequency.