Anyway to avoid the startup delay on square wave generator (e.g. 555 astable) circuit?
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Flying start 555 oscillator.

I've only used a 555 twice in the last few decades but this may do the trick for you.


C3 is connected between V+ and the timing capacitor. On power up C3 and C1 form a capacitive potential divider. If the voltage rises smartly the junction of C3 and C1 will rise to \$V_+ \frac {C_3}{C_3+C_1} \$ thus priming the capacitor for you. So, to aim for 1/2 supply set C3 = C1. Bear in mind that R1 and R2 will be discharging C3 during power up so you may have some calculations or experiments to do.
Once the oscillator starts up the timing will depend in C1 in parallel with C3 so use value \$C = {C_1+C_3} \$ in your timing calculations. The fact that C3 is connected to V+ rather than GND won't matter.