Update README.md

C64_4joy_adapter/README.md

@@ -39,6 +39,22 @@ sei
 - lds/sts takes 2 cycles and in/out takes 1 cycle? ldi takes 1 cycle?
 - lds/sts: "M" \_SFR_MEM_ADDR(GPIOR2), in/out: "I" \_SFR_IO_ADDR(GPIOR0)
 
+## interrupts
+- https://billgrundmann.wordpress.com/2009/03/02/the-overhead-of-arduino-interrupts/
+- https://forum.arduino.cc/t/how-fast-can-i-interrupt/25884/5
+```
+This is from the datasheet for the AT90USB82 processor; things in parenthesis are from me...
+
+The interrupt execution response for all the enabled AVR interrupts is five clock cycles minimum (the processor is fixin' to execute the interrupt)
+The vector is normally a jump to the interrupt routine, and this jump takes three clock cycles (the processor jumps to the ISR)
+SREG must be saved and restored (the processor doesn't do this for us and SREG is important)
+A return from an interrupt handling routine takes three clock cycles
+When the AVR exits from an interrupt, it will always return to the main program and execute one more instruction before any pending interrupt is served
+Those are the things necessary just to get the ISR called. We have not yet added the application stuff (incrementing an unsigned long in BetterSense's case).
+
+Adding those up gives us 5+3+2+3+1 = 14. The absolute maximum number of interrupts per second that can be handled by the AT90USB82 is 16 million instructions per second / 14 instructions per interrupt = 1,142,857 interrupts per second.
+```
+
 ## avr asm
 - http://www.nongnu.org/avr-libc/user-manual/inline_asm.html
 - https://ucexperiment.wordpress.com/2016/03/04/arduino-inline-assembly-tutorial-1/