2 files changed, 58 insertions, 0 deletions
diff --git a/trasmitter/README.rst.txt b/trasmitter/README.rst.txt
index 4b9e310..d2fd957 100644
--- a/trasmitter/README.rst.txt
+++ b/trasmitter/README.rst.txt
@@ -38,3 +38,12 @@ sysfs interface`_, seem to be fast enough for our purposes.
 
 See the ``sender-chip.pl`` test program.
 
+The timing via the sysfs interface is awful.
+
+It looks like we can drive the pins (not the XIO ones) via
+memory-mapped registers: the sunxi-tools ``pio.c`` has most of the
+code needed.
+
+We could write a C program to just send the signal train. Look at
+``clock.c`` for a way to handle fast / real-time scheduling and
+delays.
diff --git a/trasmitter/clock.c b/trasmitter/clock.c
new file mode 100644
index 0000000..2d8bd61
--- /dev/null
+++ b/trasmitter/clock.c
@@ -0,0 +1,49 @@
+#include <stdio.h>
+#include <time.h>
+#include <sched.h>
+
+void main(int argc,char **argv) {
+  struct timespec begin_time,end_time,next_step,end_time2;
+  struct sched_param sp;
+
+  const int width=520000;
+  const int pulses=30;
+
+  sp.sched_priority=99;
+
+  // to make this work, sudo setcap CAP_SYS_NICE=eip clock
+  sched_setscheduler(0,SCHED_RR,&sp);
+
+  clock_gettime(CLOCK_REALTIME,&begin_time);
+  clock_gettime(CLOCK_REALTIME,&next_step);
+  for (int i=0;i<pulses;++i) {
+    // do something slow
+    clock_gettime(CLOCK_REALTIME,&end_time2);
+    // end of "slow"
+
+    next_step.tv_nsec += width;
+
+    if (next_step.tv_nsec > 1000000000UL) {
+      next_step.tv_nsec -= 1000000000UL;
+      ++next_step.tv_sec;
+    }
+
+    int rc = clock_nanosleep(CLOCK_REALTIME,
+                             TIMER_ABSTIME,
+                             &next_step,
+                             NULL);
+    if (rc) {
+      printf("problems: %d\n",rc);
+    }
+  }
+  clock_gettime(CLOCK_REALTIME,&end_time);
+  clock_gettime(CLOCK_REALTIME,&end_time2);
+
+  printf("began at %ld.09%ld\n",begin_time.tv_sec,begin_time.tv_nsec);
+  printf("ended at %ld.09%ld\n",end_time.tv_sec,end_time.tv_nsec);
+  printf("ended at %ld.09%ld\n",end_time2.tv_sec,end_time2.tv_nsec);
+
+  long elapsed = end_time.tv_nsec - begin_time.tv_nsec;
+  printf("elapsed: %ld\n",elapsed);
+  printf("error: %ld\n",elapsed-(pulses*width));
+}