sys/ztimer: fix RTT backend

[jue89]

Contribution description

First of all: congratulations on merging ztimer!

While trying tests/ztimer_msg with TIMER_MSEC I noticed that ztimer ignores the present periph_rtt and converted the pertiph_timer down to milliseconds.

This patch fixes this behavior.

Testing procedure

Enable debugging in sys/ztimer/auto_init.c. Compile tests/ztimer_msg with USEMODULE="ztimer_msec ztimer_periph_rtt" make BOARD=board_with_rtt flash term.

Boot output with this patch:

020-03-06 14:05:11,078 # ztimer_init(): ZTIMER_USEC using periph timer 0, freq 1000000, width 32
2020-03-06 14:05:11,081 # ztimer_init(): initializing rtt
2020-03-06 14:05:11,087 # ztimer_init(): ZTIMER_MSEC convert_frac from 32768 to 1000
2020-03-06 14:05:11,092 # Help: Press s to start test, r to print it is ready

Boot output without this patch:

2020-03-06 14:06:13,116 # ztimer_init(): ZTIMER_USEC using periph timer 0, freq 1000000, width 32
2020-03-06 14:06:13,122 # ztimer_init(): ZTIMER_MSEC convert_frac from 1000000 to 1000
2020-03-06 14:06:13,126 # Help: Press s to start test, r to print it is ready

Issues/PRs references

#11874

[jue89]

ztimer ist evolving quickly :D Rebased on master ...

[kaspar030]

ups, I thing this happened when renaming ztimer_(periph|rtt|rtc) to ztimer_periph_(timer|rtt|rtc) with search&replace.

[kaspar030]

First of all: congratulations on merging ztimer!

thanks. :)

[kaspar030]

ACK.