After programming an ATmega microcontroller via ISP (SPI) interface and changing the clock-related fuse bits, I'd like to reprogram it again. Will I have to connect it to an external clock now to do so, or will I always be able to program it by clocking just via SPI's SCLK line, without external clock (like in image below)?
(in the image below, the programmer is a Raspberry Pi)
Hmh, found an answer to other similar question:
You do not need any crystal circuitry in place for programming a blank off the line AVR (as they come running off the internal oscillator), but if trying to reprogram a chip that is fused for an external crystal, you will need a comparable crystal and load capacitors in circuit (unless you use a high voltage programming mode).
so for now I understand that the answer is I can't use the above simple circuit after changing the fuse bits, I have to extend it with a clock matching the fuse bits configuration.
It depends on just what you set the clock source fuse bits to. The default configuration is usually to run off an internal 8MHz oscillator divided down to 1MHz; other configurations include external clock (typically on XTAL1 pin) and various crystal modes (with a crystal resonator across XTAL1 and XTAL2). In each of these modes, clock rates down to DC are supported, so the chip will be effectively frozen if it doesn't receive a clock input. However, in oscillator mode XTAL2 is essentially an inverted amplification of the XTAL1 input just to get the resonator swinging, and you could therefore simply feed a clock into XTAL1 in all cases. It will simply be ignored if the internal oscillator is used.
The two combinations that cause trouble are:
