system_timer_current_time_us drifts behind us_ticker_read at a variable rate #451
Description
system_timer_current_time_us loses time relative to us_ticker_read at a variable rate. This affects system_timer_current_time and uBit.systemTime. The drift is usually 1.5 to 4.5 ms/s, but can be higher. Very occasionally, system_timer suddenly catches up by about 60ms.
Reading the system_timer more often speeds up the drift. I suspect interrupts occurring inside update_time cause ticks to be dropped.
This could be improved by only resetting the Timer when timer->read_us() > 2000000000.
Maybe a better solution is to ditch Timer and use us_ticker_read directly?
The example below sends to serial, when button A is clicked or B held down, the times, the difference between them and the rate of drift in microseconds per second.
#include "MicroBit.h"
#include "MicroBitCompat.h"
MicroBit uBit;
uint64_t sys0;
uint64_t sys1;
uint64_t sys;
uint64_t tic0;
uint64_t tic1;
uint64_t tic;
int64_t dif;
double speed;
uint64_t uspers;
void sendTimes()
{
sys1 = system_timer_current_time_us();
tic1 = us_ticker_read();
sys = sys1 - sys0;
tic = tic1 - tic0;
// difference between us_ticker_read and system_timer_current_time_us
dif = tic >= sys ? tic - sys : -(sys - tic);
// calculate drift in us/s
speed = (double)dif / (double)tic;
uspers = speed * 1000000.0;
ManagedString st( (int)tic);
ManagedString ss( (int)sys);
ManagedString sd( (int)dif);
ManagedString sv( (int)uspers);
ManagedString s = st + "," + ss + "," + sd + "," + sv + "\n";
uBit.serial.send( s);
}
void onButtonA(MicroBitEvent)
{
sendTimes();
}
void onButtonB(MicroBitEvent)
{
while ( uBit.buttonB.isPressed())
{
uBit.sleep(10);
sendTimes();
}
}
int main()
{
uBit.init();
uBit.serial.send( "\n\ntick,sys,tick-sys,speed (us/s)\n");
sys0 = system_timer_current_time_us();
tic0 = us_ticker_read();
sys = 0;
tic = 0;
dif = 0;
uBit.serial.send( "\n\n");
uBit.messageBus.listen( MICROBIT_ID_BUTTON_A, MICROBIT_BUTTON_EVT_CLICK, onButtonA);
uBit.messageBus.listen( MICROBIT_ID_BUTTON_B, MICROBIT_BUTTON_EVT_DOWN, onButtonB);
release_fiber();
}