at85c51snd3 ATMEL Corporation, at85c51snd3 Datasheet - Page 28

• Manufacturer Part Number: at85c51snd3
• Description: At85c51snd3 Single-chip Digital Audio Decoder - Encoder With Usb 2.0 Interface
• Manufacturer: ATMEL Corporation

Clock Controller
Oscillator
Authorized frequency
Power Optimization
28
AT85C51SND3B
The clock controller implemented in AT85C51SND3B derivatives is based on an on-chip
oscillator feeding two on-chip Phase Lock Loop (PLL) dedicated for the USB controller
(see Section “USB Controller”, page 85) and the Audio Controller (see Section “Audio
Controller”, page 149). All internal clocks to the peripherals and CPU core are gener-
ated by this controller.
X1 and X2 pins are the input and the output of a frequency power-optimized single-
stage on-chip inverter (see Figure 15) that can be configured with off-chip components
such as a Pierce oscillator (see Figure 16). Value of capacitors and crystal characteris-
tics are detailed in the Section “DC Characteristics”, page 242.
In order to be able to be able to properly detect the oscillating frequency when in In Sys-
tem Programming mode and then generate the 480MHz requested for USB connection,
only the following frequencies are authorized:
12MHz, 13MHz, 16MHz, 19.2MHz, 19.5MHz, 20MHz, 24MHz and 26MHz.
In order to optimize the power consumption, oscillator gain can be adjusted by software
depending on the crystal frequency. Such optimization is done after reset using
OSCF1:0 bits in CKCON register (see Table 31) according to Table 23. Moreover if
external frequency signal is input (X1 driven by a remote host) it is possible to switch off
the internal amplifier by setting the OSCAMP bit in CKCON register as shown in
Figure 15.
Table 23. Oscillator Frequency Configuration
The oscillator outputs a clock: the oscillator clock used to feed the clock generator and
the system clock generator.
The oscillator clock can be disabled by entering the power-down reduction mode as
detailed in the Section “Power Management”, page 19.
Figure 15. Oscillator Block Diagram and Symbol
OSCF1:0
00
01
10
11
X1
X2
OSCF1:0
CKCON4:3
Crystal Clock Frequency Range (F
22 - 26 MHz (default)
18 - 22 MHz
14 - 18 MHz.
10 - 14 MHz
OSCAMP
CKCON.5
PCON.1
PD
OSC
Oscillator Clock Symbol
Oscillator
Clock
)
CLOCK
OSC
7632D–MP3–01/07