NEC uPD72257, Предварительное руководство пользователя

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4.3 Resets
The graphics controller provides the following four reset sources:
• external RESET
• user reset via the control bit SYSRESET.PLLRESET
• user reset via the control bit SYSRESET.SWRESET
• system watchdog reset WDRESET
The different resets act on different targets:
System reset A system reset is applied to all modules except the clock generator circuitry. All
registers except SYSPLLCTRL and SYSCLKCTRL take on their default values.
Consequently the clock generator circuitry is kept in operation and the main
system clock supply HCLK is maintained. That means in particular that the
graphics controller is ready to operate in very short time.
All reset sources trigger a system reset.
SYSCLKCTRL reset A reset of the SYSCLKCTRL register is initiated only by an external RESET.
Since the external RESET initializes the entire graphics controller, it comes up with
all default settings, partly defined by the MODE pins due to the boot mode
function.
PLL reset The effect of a PLL reset differs depending on the reset source:
• external RESET
An external RESET sets the SYSPLLCTRL register to its default
values, defined by MODE[5:4], and restarts the PLL with the default
parameters in SYSPLLCTRL.
• by user SYSRESET.PLLRESET = 1
This user reset restart only the PLL, but does not reset the
SYSPLLCTRL register - and hence does not apply the MODE[5:4]
default settings to the register.
Purpose of this reset is to enable the user to set up own PLL
parameters in SYSPLLCTRL and to restart the PLL with these
parameters.
Note that this reset only changes the PLL parameters - and therefore
its output frequency -, but not the values of the main system clock
divider BUSDIV[1:0] and the Video Output clocks dividers
VOnDIV[5:0] in the SYSCLKCTRL register.
The table below informs about which reset source acts on which reset target.
Table 4-1 Reset sources and targets
Reset
source
System
reset
PLL reset
SYSCLKCTRL
reset
Purpose
SYSPLLCTRL
reset
PLL
restart
RESET yes yes yes yes
complete reset with PLL/clock
setup configuration according to
boot mode pins
SYSRESET.
PLLRESET
yes no yes no
software reconfiguration of PLL
settings (after write to
SYSPLLCTRL)
System Controller Chapter 4
Preliminary User's Manual S19203EE1V3UM00
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