This schematic shows several ATtiny45 minimal circuit configurations. The ATtiny45 is shown using its internal 8 MHz RC oscillator, which divided by 8 gives a 1 MHz system clock, and with an external crystal and ADC buffer. The ATtiny45 has 4kB flash, 256 bytes of SRAM, and 256 bytes of EEPROM. There are two 8-bit timers, and no 16-bit timers.
Here you can find the current ATtiny45 datasheet
When you program the ATtiny45 in circuit the SCK line drives the analog input signal with a 0 - 5V high frequency square wave. To prevent harm to the analog attached device, a buffer amplifier is placed inline. The NE5230 is a single supply rail-to-rail device that is stable at unity gain. R2 protects the output of the amplifier, and prevents the analog input from disturbing the SCK signal during programming.