7 ClOCK GeneRaTOR (ClG)
S1C17624/604/622/602/621 TeChniCal Manual
Seiko epson Corporation
7-7
Table 7.4.2 lists the combinations of clock operating states and register settings enabling sys-
tem clock (OSC1 or HSCLK) selection.
4.2 System Clock Switching (OSC1
Table 7.
↔
HSCLK) Conditions
iOSC
OSC3
OSC1
hSClKSel
System clock
On
On
On
*
IOSC/OSC3 or OSC1
On
Off
On
0
IOSC or OSC1
Off
On
On
1
OSC3 or OSC1
• When switching the HSCLK source (IOSC
↔
OSC3), always make sure that PCKEN[1:0]/CLG_
PCLK register is set to 0x3 before writing to HSCLKSEL.
• The oscillator circuit selected as the system clock source cannot be turned off.
• Continuous write/read access to CLKSRC is prohibited. At least one instruction unrelated to
CLKSRC access must be inserted between the write and read instructions.
• Canceling HALT/SLEEP mode does not change the clock status configured before the chip
entered HALT/SLEEP mode.
CPu Core Clock (CClK) Control
7.5
The CLG module includes a clock gear to slow down the system clock to send to the S1C17 Core. To reduce cur-
rent consumption, operate the S1C17 Core with the slowest possible clock speed. The
halt
instruction can be ex-
ecuted to stop the clock supply from the CLG to the S1C17 Core for power savings.
HSCLK
OSC1
CCLK
Clock gear
(1/1–1/8)
Gate
S1C17 Core
Gear selection
System clock
HALT
5.1 CCLK Supply System
Figure 7.
Clock gear settings
CCLKGR[1:0]/CLG_CCLK register is used to select the gear ratio to reduce system clock speeds.
5.1 CCLK Gear Ratio Selection
Table 7.
CClKGR[1:0]
Gear ratio
0x3
1/8
0x2
1/4
0x1
1/2
0x0
1/1
(Default: 0x0)
Clock supply control
The CCLK clock supply is stopped by executing the
halt
instruction. Since this does not stop the system
clock, peripheral modules will continue to operate.
HALT mode is cleared by resetting, NMI, or other interrupts. The CCLK supply resumes when HALT mode is
cleared.
Executing the
slp
instruction suspends system clock supply to the CLG, thereby halting the CCLK supply as
well. Clearing SLEEP mode with an external interrupt restarts the system clock supply and the CCLK supply.
Peripheral Module Clock (PClK) Control
7.6
The CLG module also controls the clock supply to peripheral modules.
The system clock is used unmodified for the peripheral module clock (PCLK).