pulseIn() 21 cycles must be reevaluated with Arduino 1.5.8

[jmparatte]

Hi, I'm playing with Arduino UNO and Seeeduino Lite (Leonardo equivalent), compiling with Arduino 1.5.8. I've observed that the measured pulse is about 20% shorter than expected. Examining the code, I count 25 cycles rather the 21 cycles produced with Arduino 1.0.6.
My forum thread: http://forum.arduino.cc/index.php?topic=290248.0

[cmaglie]

@jmparatte
thanks for the analysis!

In your disassemble I found an error on the cpse instruction, the documentation http://www.atmel.com/images/doc0856.pdf says that for cpse:

Cycles:
1 if condition is false (no skip)
2 if condition is true (skip is executed) and the instruction skipped is 1 word
3 if condition is true (skip is executed) and the instruction skipped is 2 words

since cpse always skips the rjmp instruction (except on the last loop) it takes 2 cycles.
The rjmp that follows takes 0 cycles (being skipped).
So, in conclusion we have the following:

                   .L8:
00ae FB01                  movw r30,r22  ; 1 cycle
00b0 E590                  lpm r14, Z+   ; 3 cycles
00b2 F490                  lpm r15, Z    ; 3 cycles
00b4 F701                  movw r30,r14  ; 1 cycle
00b6 F080                  ld r15,Z      ; 2 cycles
00b8 FD22                  and r15,r29   ; 1 cycle
00ba FC12                  cpse r15,r28  ; 2 cycles (uses 2 cycles: skips rjmp that is 1 word)
00bc 00C0                  rjmp .L15     ; 0 cycles (uses 0 cyles: skipped from cpse)
                   .LM13:
00be 2817                  cp r18,r24    ; 1 cycle
00c0 3907                  cpc r19,r25   ; 1 cycle
00c2 4A07                  cpc r20,r26   ; 1 cycle
00c4 5B07                  cpc r21,r27   ; 1 cycle
00c6 01F0                  breq .L13     ; 1 cycle
                   .LM14:
00c8 2F5F                  subi r18,-1   ; 1 cycle
00ca 3F4F                  sbci r19,-1   ; 1 cycle
00cc 4F4F                  sbci r20,-1   ; 1 cycle
00ce 5F4F                  sbci r21,-1   ; 1 cycle
00d0 00C0                  rjmp .L8      ; 2 cycle

for a total of 24.
I've implemented the patch here: #2556