ST10F276 datasheet(198/229 Pages) STMICROELECTRONICS

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Electrical characteristics

ST10F276

198/229

Due to this adaptation to the input clock, the frequency of fCPU is constantly adjusted so it is

locked to fXTAL. The slight variation causes a jitter of fCPU which also effects the duration of

individual TCLs.

The timings listed in the AC Characteristics that refer to TCLs therefore must be calculated

using the minimum TCL that is possible under the respective circumstances.

The real minimum value for TCL depends on the jitter of the PLL. The PLL tunes fCPU to

keep it locked on fXTAL. The relative deviation of TCL is the maximum when it is referred to

one TCL period.

This is especially important for bus cycles using wait states and e.g. for the operation of timers,

serial interfaces, etc. For all slower operations and longer periods (such as, for example, pulse

train generation or measurement, lower baud rates) the deviation caused by the PLL jitter is

negligible. Refer to next Section 23.8.9: PLL Jitter for more details.

23.8.8

Voltage controlled oscillator

The ST10F276 implements a PLL which combines different levels of frequency dividers with

a Voltage Controlled Oscillator (VCO) working as frequency multiplier. The following table

presents a detailed summary of the internal settings and VCO frequency.

The PLL input frequency range is limited to 1 to 3.5 MHz, while the VCO oscillation range is

64 to 128 MHz. The CPU clock frequency range when PLL is used is 16 to 64 MHz.

Example 1

–FXTAL = 4 MHz

–

P0(15:13) = ‘110’ (multiplication by 3)

–

PLL input frequency = 1 MHz

–

VCO frequency = 48 MHz

–

PLL output frequency = 12 MHz

(VCO frequency divided by 4)

–FCPU = 12 MHz (no effect of output prescaler)

Table 96.

Internal PLL divider mechanism

P0.15-13

(P0H.7-5)

XTAL

frequency

Input

prescaler

PLL

Output

prescaler

CPU frequency

fCPU = fXTAL x F

Multiply by

Divide by

14 to 8 MHz

FXTAL / 4

–

FXTAL x 4

5.3 to

10.6 MHz

FXTAL / 4

–

FXTAL x 3

14 to 8 MHz

FXTAL / 4

–

FXTAL x 8

06.4 to 12 MHz FXTAL / 4

–

FXTAL x 5

11 to 64 MHz

–

PLL bypassed

–

FXTAL x 1

04 to 6.4 MHz

FXTAL / 2

–

FXTAL x 10

4 to 12 MHz

–

PLL bypassed

FPLL / 2

FXTAL / 2

04 MHz

FXTAL / 2

–

FXTAL x 16

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