at89c5131 ATMEL Corporation, at89c5131 Datasheet - Page 20

• Manufacturer Part Number: at89c5131
• Description: 8-bit Flash Microcontroller With Full Speed Usb Device At89c5131
• Manufacturer: ATMEL Corporation

PLL
PLL Description
Figure 7. PLL Block Diagram and Symbol
20
AT89C5131
CLOCK
OSC
N divider
N3:0
Figure 6. Crystal Connection
The AT89C5131 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 7 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 coming 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 7) is set.
The CHP block is the Charge Pump that generates the voltage reference for the VCO by
injecting or extracting charges from the external filter connected on PLLF pin (see
Fi gure 8) . Value of the filter components ar e detailed in the Section “ DC
Characteristics”.
The VCO block is the Voltage Controlled Oscillator controlled by the voltage V
duced by the charge pump. It generates a square wave signal: the PLL clock.
Figure 8. PLL Filter Connection
The typical values are: R = 100 Ω, C1 = 10 nf, C2 = 2.2 nF.
PLLCON.1
PLLCON.0
PLOCK
PLLEN
USBclk
PFLD
Down
=
Up
OSCclk
---------------------------------------------- -
R divider
R3:0
PLLF
CHP
PLLF
N
VSS
+
×
1
(
R
+
Vref
C1
C2
1
)
VSS
R
VCO
C1
Q
VSS
X1
X2
C2
USB Clock Symbol
USB Clock
CLOCK
USB
4136C–USB–04/05
REF
pro-