Datasheet

4341H–MP3–10/07

AT8xC51SND2C/MP3B

5.2 X2 Feature

Unlike standard C51 products that require 12 oscillator clock periods per machine cycle, the

AT8xC51SND2C need only 6 oscillator clock periods per machine cycle. This feature called the

“X2 feature” can be enabled using the X2 bit

(1)

in CKCON (see Table 5-1) and allows the

AT8xC51SND2C to operate in 6 or 12 oscillator clock periods per machine cycle. As shown in

Figure 5-1, both CPU and peripheral clocks are affected by this feature. Figure 5-3 shows the X2

mode switching waveforms. After reset the standard mode is activated. In standard mode the

CPU and peripheral 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 6-3

on page 22). Using the AT89C51SND2C (Flash Version) the system can boot either in stan-

dard or X2 mode depending on the X2B value. Using AT83SND2C (ROM Version) the system

always boots in standard mode. X2B bit can be changed to X2 mode later by software.

Figure 5-3. Mode Switching Waveforms

Note: 1. In order to prevent any incorrect operation while operating in X2 mode, user must be aware

that all peripherals using clock frequency as time reference (timers, etc.) will have their time

reference divided by 2. For example, a free running timer generating an interrupt every 20 ms

will then generate an interrupt every 10 ms.

5.3 PLL

5.3.1 PLL Description

The AT8xC51SND2C PLL is used to generate internal high frequency clock (the PLL Clock) syn-

chronized with an external low-frequency (the Oscillator Clock). The PLL clock provides the MP3

decoder, the audio interface, and the USB interface clocks. Figure 5-4 shows the internal struc-

ture 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 5-2) 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 5-5). Value

of the filter components are detailed in the Section “DC Characteristics”.

The VCO block is the Voltage Controlled Oscillator controlled by the voltage V

ref

produced by the

charge pump. It generates a square wave signal: the PLL clock.

X1 ÷ 2

Clock

X2 Bit

X2 Mode

(1)

STD Mode STD Mode