atmega32u4-16mu ATMEL Corporation, atmega32u4-16mu Datasheet - Page 38

• Manufacturer Part Number: atmega32u4-16mu
• Description: Atmega32u4 8-bit Avr Microcontroller With 32k Bytes Of Isp Flash And Usb Controller
• Manufacturer: ATMEL Corporation

6.9
6.9.1
6.9.2
38
PLL
ATmega32U4
Internal PLL
PLL Control and Status Register – PLLCSR
The PLL is used to generate internal high frequency (up to 96MHz) clock for USB interface
and/or High Speed Timer module, the PLL input is supplied from an external low-frequency
clock (the crystal oscillator or external clock input pin from XTAL1).
The internal PLL in ATmega32U4 generates a clock frequency between 32MHz and 96 MHz
from nominally 8MHz input.
The source of the 8MHz PLL input clock is the output of the internal PLL clock prescaler that
generates the 8MHz from the clock source multiplexer output (See
face). The PLL prescaler allows a direct connection (8MHz oscillator) or a divide-by-2 stage for a
16MHz clock input.
The PLL output signal enters the PLL Postcaler stage before being distributed to the USB and
High Speed Timer modules. Each of these modules can choose an independent division ratio.
Figure 6-4.
• Bit 7..5 – Res: Reserved Bits
These bits are reserved bits in the ATmega32U4 and always read as zero.
• Bit 4 – PINDIV PLL Input Prescaler (1:1, 1:2)
These bits allow to configure the PLL input prescaler to generate the 8MHz input clock for the
PLL from either a 8 or 16 MHz input.
When using a 8 MHz clock source, this bit must be set to 0 before enabling PLL (1:1).
When using a 16 MHz clock source, this bit must be set to 1 before enabling PLL (1:2).
• Bit 3..2 – Res: Reserved Bits
These bits are reserved bits in the ATmega32U4 and always read as zero.
XTAL1
XTAL2
Bit
$29 ($29)
Read/Write
Initial Value
RC OSCILLATOR
8 MHz
OSCILLATOR
XTAL
7
R
0
PLL Clocking System
6
R
0
CKSEL3:0
5
R
0
To System
Clock Prescaler
PLL clock
Prescaler
PINDIV
4
PINDIV
R/W
0
3
R
0
PINMUX
0
1
clk
8MHz
2
R
0
PLOCK
Detector
Lock
PDIV3..0
PLL
PLLE
1
PLLE
R/W
0
Section 6.9.2
/1.5
0
PLOCK
R
0
/2
7766A–AVR–03/08
PLLTM1:0
for PLL inter-
PLLUSB
10
11
01
1
0
PLLCSR
clk TMR
clk USB