Datasheet

15
4337K–USB–04/08
AT89C5130A/31A-M
In order to optimize the power consumption, the oscillator inverter is inactive when the PLL out-
put is not selected for the USB device.
Figure 5-2. Crystal Connection
5.3 PLL
5.3.1 PLL Description
The AT89C5130A/31A-M PLL is used to generate internal high frequency clock (the USB Clock)
synchronized with an external low-frequency (the Peripheral Clock). The PLL clock is used to
generate the USB interface clock. Figure 5-3 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 Figure 5-3) 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 PLLF pin (see Figure 5-4). Value
of the filter components are detailed in the Section “DC Characteristics”.
The VCO block is the Voltage Controlled Oscillator controlled by the voltage V
REF
produced by
the charge pump. It generates a square wave signal: the PLL clock.
Figure 5-3. PLL Block Diagram and Symbol
VSS
X1
X2
Q
C1
C2
PLLEN
PLLCON.1
N3:0
N divider
R divider
VCO USB Clock
USBclk
OSCclk R 1+( )×
N 1+
-----------------------------------------------=
OSC
CLOCK
PFLD
PLOCK
PLLCON.0
PLLF
CHP
Vref
Up
Down
R3:0
USB
CLOCK
USB Clock Symbol