AT89C5132
eral clock frequency is the oscillator frequency divided by 2 while in X2 mode, it is the oscillator
frequency.
Note:
1. The X2 bit reset value depends on the X2B bit in the Hardware Security Byte (see Table 12 on
page 24). Using the AT89C5132 (Flash Version) the system can boot either in standard or X2
mode depending on the X2B value. Using AT83C51SND1C (ROM Version) the system always
boots in standard mode. X2B bit can be changed to X2 mode later by software.
Figure 6-3. Mode Switching Waveforms
X1
X1 ÷ 2
X2 Bit
Clock
X2 Mode(1)
STD Mode
STD Mode
Note:
In order to prevent any incorrect operation while operating in X2 mode, the user must be aware
that all peripherals using clock frequency as time reference (timers…) will have their time refer-
ence divided by two. For example, a free running timer generating an interrupt every 20 ms will
then generate an interrupt every 10 ms.
6.3
PLL
6.3.1
PLL Description
The AT89C5132 PLL is used to generate internal high frequency clock (the PLL Clock) synchro-
nized with an external low-frequency (the Oscillator Clock). The PLL clock provides the audio
interface, and the USB interface clocks. Figure 6-4 shows the internal structure of the PLL.
The PFLD block is the Phase Frequency Comparator and Lock Detector. This block makes the
comparison between the reference clock coming from the N divider and the reverse clock com-
ing from the R divider and generates some pulses on the Up or Down signal depending on the
edge position of the reverse clock. The PLLEN bit in PLLCON register is used to enable the
clock generation. When the PLL is locked, the bit PLOCK in PLLCON register (see Table 3) is
set.
The CHP block is the Charge Pump that generates the voltage reference for the VCO by inject-
ing or extracting charges from the external filter connected on PFILT pin (see Figure 6-5). Value
of the filter components are detailed in the Section “DC Characteristics”.
The VCO block is the Voltage Controlled Oscillator controlled by the voltage Vref produced by the
charge pump. It generates a square wave signal: the PLL clock.
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4173E–USB–09/07
ATMEL [ ATMEL ]
