The gap is just as long as when waiting for an event but the "start bit"
on DAT0 has vanished completely:
1 3 gap 2 0 1 3
------- ------- ------- ------- ------- ------- -------
53 77 16 146
66 79 1 146
81 79 132
96 79 117
This seems to make no difference for the gap but the "start bit" (DAT0
pulled low) seem to get 1-2 samples shorter:
1 3 gap 2 0 1 3
------- ------- ------- ------- ------- ------- -------
146 77 9 60
147 13 79 9 45
145 28 79 10 29
146 43 79 9 15
A series of measurements of
A# ./ubb-patgen -f 41kHz 1
A# ./ubb-patgen -f 41kHz -c
B# ./ubb-la -f 12 -n 10
yielded these results:
1 3 gap 2 0 1 3
------- ------- ------- ------- ------- ------- -------
106 77 11 98
120 78 11 83
134 79 11 68
3 79 11 53
18 78 11 39
33 78 11 24
47 79 11 9
62 79 5 6 140 147
77 79 11 125 147
Where for example the last entry corresponds to
...1{146}3{77}
0{11}1{125}3{147}...
Since this looks as if DAT1 was 1 for 77 samples before the first capture
ended, was 0 throughout the pulling low of DAT0 (11 cycles), stayed low
for another 125 cycles, and then went high for the 146.29 nominal
half-period, we thus get a gap length of 2*146-77-11-125 = 79
We inherited the cast from ubb-patgen where the buffer was "const" and thus
had to be cast for the the non-const argument of physmem_xlat. We never
needed a cast in ubb-la, though.
Since physmem_xlat now uses "const" as well, the cast is even doubly
superfluous.
Third time lucky, I hope. -fno-tree-cselim is much more specific than
disabling all optimization and results in a considerably less severe
performance reduction (about 30-40% of -O0).
While -O1 gets rid of the unexpected read in the simple code of a synthetic
test, it's still there in the more complex environment we have in ubb-la.c
Turning off optimization completely seems to do the trick.
Note that the pull-ups on DAT1 through DAT3 and the pull-whichever-way on
DAT0 are likely to get in the way of any real-life use. But it's good enough
for exploring the system's characteristics and limitations.
