S12 Clock, Reset and Power Management Unit (S12CPMU_UHV_V8)
MC9S12VRP Family Reference Manual Rev. 1.3
160
NXP Semiconductors
4.4.6.3
PLL Bypassed External Mode (PBE)
In this mode, the Bus Clock frequency is half the external oscillator clock. The reference clock for the PLL
is based on the external oscillator.
The clock sources for COP and RTI can be based on the internal reference clock generator or on the
external oscillator clock or the RC-Oscillator (ACLK).
This mode can be entered from default mode PEI by performing the following steps:
1. Make sure the PLL configuration is valid.
2. Enable the external Oscillator (OSCE bit)
3. Wait for the oscillator to start-up and the PLL being locked (LOCK = 1) and (UPOSC =1)
4. Clear all flags in the CPMUFLG register to be able to detect any status bit change.
5. Optionally status interrupts can be enabled (CPMUINT register).
6. Select the Oscillator clock as source of the Bus Clock (PLLSEL=0)
Loosing PLL lock status (LOCK=0) means loosing the oscillator status information as well (UPOSC=0).
The impact of loosing the oscillator status (UPOSC=0) in PBE mode is as follows:
•
PLLSEL is set automatically and the Bus clock is switched back to the PLL clock.
•
The PLLCLK is derived from the VCO clock (with its actual frequency) divided by four until the
PLL locks again.
NOTE
Application software needs to be prepared to deal with the impact of loosing the oscillator status at
any time.
When using the oscillator clock as system clock (write PLLSEL = 0) it is
highly recommended to enable the oscillator clock monitor reset feature
(write OMRE = 1 in CPMUOSC2 register). If the oscillator monitor reset
feature is disabled (OMRE = 0) and the oscillator clock is used as system
clock, the system might stall in case of loss of oscillation.
4.5
Resets
4.5.1
General
All reset sources are listed in
. Refer to MCU specification for related vector addresses and
priorities.
Table 4-31. Reset Summary
Reset Source
Local Enable
Power-On Reset (POR)
None
Low Voltage Reset (LVR)
None
External pin RESET
None
Illegal Address Reset
None
Summary of Contents for MC9S12VRP64
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Page 236: ...S12S Debug Module S12DBGV2 MC9S12VRP Family Reference Manual Rev 1 3 236 NXP Semiconductors ...
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