Clock Configuration - Samsung S3C2500B User Manual

S3C2500B

4.7 CLOCK CONFIGURATION

The S3C2500B has four PLL clocking scheme – CPU PLL, System BUS PLL, USB PLL, PHY PLL. All of the PLL
can operate if the corresponding clock select pin is set to "0" (CLKSEL- shared with CPU PLL and System BUS
PLL, USB_CLKSEL, PHY_CLKSEL). When the clock select pin is set to "1", the PLL goes into power down state.
The CPU PLL can generate ARM940T clock or system bus clock depending on clock mode selection
(CLKMOD[1:0]). The System BUS PLL generates system bus clock only. The USB PLL generates USB clock.
The PHY PLL generates clock for external devices. Each PLL clock output frequency can be programmed by
either the pin setting or software setting. In pin configurable mode, the CPU_FREQ[2:0] pins determine the
frequency of the CPU PLL clock output and the BUS_FREQ[2:0] pins determine the frequency of the System
BUS PLL clock output. The USB PLL always generates 4.8 times the input clock, i.e. if the 10MHz input clock is
provided, the USB clock output is always 48 MHz. The PHY_FREQ pin determines the frequency of the PHY PLL
output. The PHY PLL generates 2 times the input clock if the PHY_FREQ is "0" and 2.5 times the input clock if
the PHY_FREQ is "1".
The CLKMOD[1:0] pins determine the relation of the ARM940T clock and the system clock. If the CLKMOD[1:0]
is "00", the fastbus clock mode is defined. In this mode, the ARM940T clock and the system bus clock is the
same clock and the clock is from the CPU PLL output. The two clocks are of the same phase and of the same
frequency. If the CLKMOD[1:0] is "11", the async clock mode is defined. In this mode, the ARM940T clock is out
of the CPU PLL and the system bus clock is out of the System BUS PLL. The frequency of the two clocks could
be set to any frequency so long as the the frequency of the ARM940T clock is faster than that of the system bus
clock. The table 4-3 shows the clock configuration of the external pin setting.
Table 4-3. Clock Frequencies for CLKMOD Pins, CPU_FREQ Pins, and BUS_FREQ Pins
CLKMOD [1:0] CPU_FREQ [2:0] BUS_FREQ [2:0] ARM940T Clock
2'b00 (Fastbus)
2'b11 (Async)
3'b000 3'bxxx
3'b001 3'bxxx
3'b010 3'bxxx
3'b011 3'bxxx
3'b100 3'bxxx
3'b101 3'bxxx
3'b110 3'bxxx
3'b111 3'bxxx
3'b000 3'b000
3'b000 3'b001
3'b000 3'b010
3'b000 3'b011
3'b000 3'b100
3'b000 3'b101
3'b000 3'b110
3'b000 3'b111
Frequency
166MHz
150MHz
133MHz
125MHz
100MHz
66MHz
50MHz
33MHz
166MHz
166MHz
166MHz
166MHz
166MHz
166MHz
166MHz
166MHz
Not supported
SYSTEM CONFIGURATION
AMBA BUS Clock USB Clock
Frequency Frequency
166MHz
150MHz
133MHz
125MHz
100MHz
166MHz
50MHz
33MHz
133MHz
133MHz
133MHz
125MHz
100MHz
66MHz
50MHz
33MHz
48MHz
48MHz
48MHz
48MHz
48MHz
48MHz
48MHz
48MHz
48MHz
48MHz
48MHz
48MHz
48MHz
48MHz
48MHz
48MHz
4-9