HD404459 Datasheet, PDF (42/122 Pages) Renesas Technology Corp – 4-bit HMCS400-series microcomputers

HD404459 Series
Selection of Division Ratio
Division Ratio of the System Clock: 1/4, 1/8, 1/16, or 1/32 division ratio of the system clock can be
selected by setting bits 0 and 1 (SSR20 and SSR21) of system clock select register 2 (SSR2: $02A). The
values of SSR20 and SSR21 become valid when entering the watch mode after making the ratio selection.
(However, the value of SSR2 becomes valid immediately after the selection.) Therefore, when changing the
division ratio, the system clock must be stopped. There are two methods for selecting the division ratio of
the system clock as follows.
• Division ratio is selected by writing to SSR20 and SSR21 in active mode. The selected values of SSR20
and SSR21 are valid before the MCU enters watch mode. The division ratio of the system clock
becomes the written value when the MCU returns to the active mode from the watch mode.
• Division ratio is selected by writing to SSR20 and SSR21 in subactive mode. The division ratio of the
system clock becomes the selected value when the MCU returns to active mode after entering watch
mode.
Note: SSR2 is cleared in the reset and stop modes. Therefore, 1/4 division ratio of the system clock is
selected when the MCU returns from stop mode after reset.
Division Ratio of the Subsystem Clock: 1/4 or 1/8 division ratio of the subsystem clock can be selected
by setting bit 2 (SSR12) of system clock select register 1 (SSR1: $029). The value of SSR12 becomes valid
immediately after the ratio selection. When the value of SSR12 is changed, the MCU must be in active
mode. If the value of SSR12 is changed in subactive mode, the MCU may malfunction.
40