6 CLOCK MANAGEMENT UNIT (CMU)
S1C33L26 TECHNICAL MANUAL
Seiko Epson Corporation
6-9
System Clock Settings
6.6
OSC3 clock
OSC1 clock
PLL clock
CLKSEL[1:0]
SYSCLKDIV[2:0]
CLKSRC
OSC
SYSCLK
SDCLK
MCLK
Divider
(1/1–1/32)
Divider
(1/2)
6.1 System Clock Control Circuit
Figure 6.
System Clock Source Selection
6.6.1
The system clock source can be selected from OSC3, OSC1, or PLL using CLKSEL[1:0]/CMU_OSCSEL register.
6.1.1 System Clock Source Selections
Table 6.
CLKSEL[1:0]
Clock source
0x3
Reserved
0x2
PLL
0x1
OSC1
0x0
OSC3
(Default: 0x0)
The following shows system clock switching procedures:
Switching the system clock to OSC1 from OSC3
1. Turn the OSC1 oscillator on if it is off. (OSC1EN = 1)
2. Wait until the OSC1 oscillation is stabilized.
3. Stop the peripheral circuits being currently operated.
4. Select the OSC1 clock as the system clock. (CLKSEL[1:0] = 0x1)
5. Check if CLKSEL[1:0] is set to 0x1 to confirm that the system clock has been switched to OSC1.
6. Turn the OSC3 oscillator off to reduce current consumption if the CMU_CLK output circuit has not used
the OSC3 clock. (OSC3EN = 0)
Switching the system clock to PLL from OSC3
1. Configure the PLL input clock and the PLL parameters such as the multiplication rate before activating the
PLL.
2. Enable the PLL. (PLLPOWR = 1)
3. Wait until the PLL operation is stabilized.
4. Stop the peripheral circuits being currently operated except the RTC.
5. Select the PLL clock as the system clock. (CLKSEL[1:0] = 0x2)
6. Check if CLKSEL[1:0] is set to 0x2 to confirm that the system clock has been switched to PLL.
Switching the system clock to OSC3 from OSC1
1. Turn the OSC3 oscillator on if it is off. (OSC3EN = 1)
2. Wait until the OSC3 oscillation is stabilized.
3. Stop the peripheral circuits being currently operated except the RTC.
4. Select the OSC3 clock as the system clock. (CLKSEL[1:0] = 0x0)
5. Check if CLKSEL[1:0] is set to 0x0 to confirm that the system clock has been switched to OSC3.
Switching the system clock to PLL from OSC1
1. Switch the system clock to OSC3 from OSC1 by following the procedure shown above.
2. Switch the system clock to PLL from OSC3 by following the procedure shown above.