
7 CLOCK GENERATOR (CLG)
7-10
Seiko Epson Corporation
S1C17705 TECHNICAL MANUAL
D[1:0]
CLKSRC[1:0]: System Clock Source Select Bits
Select the system clock source.
8.2 System Clock Selection
Table 7.
CLKSRC[1:0]
System clock source
0x3
Reserved
0x2
OSC3
0x1
OSC1
0x0
IOSC
(Default: 0x0)
Select IOSC or OSC3 for normal (high-speed) operations. If no high-speed clock is required, OSC1 can
be set as the system clock and IOSC and OSC3 stopped to reduce current consumption.
Notes: The oscillator to be used as the system clock source must be operated before switching
the system clock. Otherwise, the CLG will not switch the system clock source, even if CLK-
SRC[1:0] is written to, and the CLKSRC[1:0] value will remain unchanged.
The table below lists the combinations of clock operating status and register settings en-
abling system clock selection.
8.3 System Clock Switching Conditions
Table 7.
IOSCEN
OSC3EN
EXOSC3EN
OSC1EN
EXOSC3SEL
System clock
1
*
IOSC, OSC3/EXOSC3, or OSC1
1
0
1
0
IOSC, OSC3, or OSC1
1
0
1
IOSC, EXOSC3, or OSC1
1
0
1
*
IOSC or OSC1
0
1
*
OSC3/EXOSC3 or OSC1
0
1
0
1
0
OSC3 or OSC1
0
1
EXOSC3 or OSC1
The oscillator circuit selected as the system clock source cannot be turned off.
Continuous write/read access to CLKSRC[1:0] is prohibited. At least one instruction unre-
lated to CLKSRC[1:0] access must be inserted between the write and read instructions.
When SLEEP mode is cancelled, the IOSC oscillator circuit is turned on (IOSCEN = 1) and
is used as the system clock source (CLKSRC[1:0] = 0x0) regardless of the system clock
configured before the chip entered SLEEP mode.
Canceling HALT mode does not change the clock status configured before the chip entered
HALT mode.
Oscillation Control Register (CLG_CTL)
Register name Address
Bit
Name
Function
Setting
Init. R/W
Remarks
Oscillation
Control Register
(CLG_CTL)
0x5061
(8 bits)
D7–6 IOSCWT[1:0] IOSC wait cycle select
IOSCWT[1:0]
Wait cycle
0x0 R/W
0x3
0x2
0x1
0x0
8 cycles
16 cycles
32 cycles
64 cycles
D5–4 OSC3WT[1:0] OSC3 wait cycle select
OSC3WT[1:0]
Wait cycle
0x0 R/W
0x3
0x2
0x1
0x0
128 cycles
256 cycles
512 cycles
1024 cycles
D3
EXOSC3EN External OSC3 enable
1 Enable
0 Disable
0
R/W
D2
IOSCEN
IOSC enable
1 Enable
0 Disable
1
R/W
D1
OSC1EN
OSC1 enable
1 Enable
0 Disable
0
R/W
D0
OSC3EN
OSC3 enable
1 Enable
0 Disable
0
R/W
D[7:6]
IOSCWT[1:0]: IOSC Wait Cycle Select Bits
An oscillation stabilization wait time is set to prevent malfunctions due to unstable clock operations at
the start of IOSC oscillation.
The IOSC clock is not supplied to the system immediately after IOSC oscillation starts until the time
set here has elapsed.