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OSCILLATOR CIRCUITS
S3C72P9/P72P9 (Preliminary Spec)
POWER CONTROL REGISTER (PCON)
The power control register, PCON, is a 4-bit register that is used to select the CPU clock frequency and to control
CPU operating and power-down modes. PCON can be addressed directly by 4-bit write instructions or indirectly by
the instructions IDLE and STOP.
FB3H
PCON.3
PCON.2
PCON.1
PCON.0
PCON bits 3 and 2 are addressed by the STOP and IDLE instructions, respectively, to engage the idle and stop
power-down modes. Idle and stop modes can be initiated by these instruction despite the current value of the enable
memory bank flag (EMB). PCON bits 1 and 0 are used to select a specific system clock frequency. There are two
basic choices:
— Main system clock (fx) or subsystem clock (fxt);
— Divided fx/4, 8, 64 or fxt/4 clock frequency.
PCON.1 and PCON.0 settings are also connected with the system clock mode control register, SCMOD. If
SCMOD.0 = "0" the main system clock is always selected by the PCON.1 and PCON.0 setting; if SCMOD.0 =
"1" the subsystem clock is selected.
RESET sets PCON register values (and SCMOD) to logic zero: SCMOD.3 and SCMOD.0 select the main system
clock (fx) and start clock oscillation; PCON.1 and PCON.0 divide the selected fx frequency by 64, and PCON.3 and
PCON.2 enable normal CPU operating mode.
Table 6-1. Power Control Register (PCON) Organization
PCON Bit Settings
Resulting CPU Operating Mode
PCON.3
PCON.2
0
0
Normal CPU operating mode
0
1
Idle power-down mode
1
0
Stop power-down mode
PCON Bit Settings
Resulting CPU Clock Frequency
PCON.1
PCON.0
If SCMOD.0 = "0"
If SCMOD.0 = "1"
0
0
fx/64
fxt/4
1
0
fx/8
1
1
fx/4
+ PROGRAMMING TIP — Setting the CPU Clock
To set the CPU clock to 0.95 µs at 4.19 MHz:
6-4
BITS
EMB
SMB
15
LD
A,#3H
LD
PCON,A