npcx: i2c: Fix i2c freq setting when APB clock is 15 MHz

To configure 1 MHz speed when the APB clock is 15 MHz, the firmware
currently sets the SCLHT register to 4. However, we found out that
writing 4 to this register (and to SCLLT register) is illegal and
results in unexpected results.  So there is a need to write 5 in that
case. However, this means that the actual i2c frequency will be 750 KHz.
To get a higher i2c clock frequency, there is a need to run with a
higher APB clock (and a higher core clock). For example, with APB set to
20 MHz, the i2c clock frequency is 833 KHz.
In this CL, the i2c freq setting for APB clock=20 MHz is also added which
may be used for NPCX7 in the future.

BRANCH=none
BUG=chromium:714314
TEST=No build error for make buildall(except gru). Use scope to capture
SCL signal on npcx5 EVB and make sure its freqency is about 750 KHz.

Change-Id: I9025344e6df4b584b203c8c59bb9875250d9fe4f
Signed-off-by: CHLin <CHLIN56@nuvoton.com>
Reviewed-on: https://chromium-review.googlesource.com/484202
Commit-Ready: Randall Spangler <rspangler@chromium.org>
Tested-by: Randall Spangler <rspangler@chromium.org>
Reviewed-by: Randall Spangler <rspangler@chromium.org>
Reviewed-by: Mulin Chao <mlchao@nuvoton.com>

0 files changed, 0 insertions, 0 deletions