4.2 Clock Generator
By use of the external resonator, connected to the XT1/XT2 pins, the oscillator
generates the clock CLKIN, that is input to the spread spectrum PLL.
The PLL output clock PLLCLKOUT is the source of various clocks, provided to
other modules:
•
the main system clock HCLK, with its frequency as a fraction of
f
PLLCLKOUT
by use of the HCLK divider
•
the Video Output clocks VO0CLK/VO1CLK, with its frequency as a
fraction of f
PLLCLKOUT
by use of the VO0CLK/VO1CLK dividers
•
the clock of the internal RAM IRAMCLK with the frequency f
HCLK
, that
can be en-/disabled by a control bit in SYSCLKCTRL
XT1/XT2
PLLCLKOUT
PLL
parameters
SYSBOOTMODE.
BOOTMODE4/8
SYSBOOTMODE.
BOOTMODE[5:4]
CLKIN
Oscillator
PLL
SYSCLKCTRL
SYSPLLCTRL
V
O0DIV[5:0]
VO0CLK divider
VO1CLK divider
HCLK divider
V
O1DIV[5:0]
B
USDIV[1:0]
HCLK
IRAMCLK
VO0CLK
VO1CLK
IR
AMCKEN
IRAM clock
control
Figure 4-2 Clock Generator
The entire clock generator is controlled by means of the SYSPLLCTRL and
SYSCLKCTRL registers.
Boot mode
The default values, i.e. the values after release of the external RESET, are subject
to the boot mode function. Thus one out of four default settings for SYSPLLCTRL
and SYSCLKCTRL are chosen, depending on the MODE[5:4] pins upon RESET
release. By this the graphics controller can be started up with some useful
configurations without the need to configure the clock generator.
The default settings comprise
•
the PLL output clock frequency f
PLLCLKOUT
•
dithering of PLLCLKOUT
•
the main system clock frequency f
HCLK
•
the pixel clock of both Video Outputs f
VO0CLK
, f
VO1CLK
The internal RAM clock IRAMCLK remains disabled in either case after RESET
release.
For further information concerning the boot mode function refer to the concerned
chapter
"Boot Mode Function"
.
Chapter 4
System Controller
88
Preliminary User's Manual S19203EE1V3UM00
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