6 CLOCK MANAGEMENT UNIT (CMU)
6-10
Seiko Epson Corporation
S1C33L26 TECHNICAL MANUAL
Switching the system clock to OSC3 from PLL
1. Stop the peripheral circuits being currently operated except the RTC.
2. Select the OSC3 clock as the system clock. (CLKSEL[1:0] = 0x0)
3. Check if CLKSEL[1:0] is set to 0x0 to confirm that the system clock has been switched to OSC3.
4. Disable the PLL to reduce current consumption if the CMU_CLK output circuit has not used the PLL
clock. (PLLPOWR = 0)
Switching the system clock to OSC1 from PLL
1. Switch the system clock to OSC3 from PLL by following the procedure shown above.
2. Switch the system clock to OSC1 from OSC3 by following the procedure shown above.
Note: Do not select the system clock from deactivated clock sources. It will cause the system to hang as
the CMU does not include a protection mechanism against such system clock selection.
System Clock Frequency Setting
6.6.2
The source clock frequency can be divided by 1 to 32 to generate the system clock using SYSCLKDIV[2:0]/CMU_
SYSCLKDIV register. Setting the system clock to the lowest frequency possible according to the processing can
reduce current consumption.
6.2.1 System Clock Division Ratio
Table 6.
SYSCLKDIV[2:0]
Division ratio (OSC/n)
0x7–0x6
1/1
0x5
1/32
0x4
1/16
0x3
1/8
0x2
1/4
0x1
1/2
0x0
1/1
(Default: 0x0)
Main System Clock (MCLK) Setting
6.6.3
The MCLK clock is the main system clock for the C33 PE Core and internal modules. It is used as CCLK, BCLK,
GCLK, PCLK1, and PCLK2.
Either SYSCLK (configured with CLKSEL[1:0] and SYSCLKDIV[2:0]) or SYSCLK/2 can be selected as MCLK
using MCLKDIV/CMU_SYSCLKDIV register.
6.3.1 MCLK (SYSCLK Division Ratio) Selections
Table 6.
MCLKDIV
MCLK (SYSCLK/n)
1
1/2
0
1/1
(Default: 0x0)
When using the SDRAMC in double frequency mode (MCLK : SDCLK = 1 : 2), MCLK should be set to
SYSCLK/2 (SYSCLK is used for the SDRAM clock).
MCLK can be selected at any time. However, up to 2 clock cycles are required before the clocks are actually
changed after altering the register values.