PC87570 datasheet(76/168 Pages) NSC

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High Frequency Clock Generator (HFCG)

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7.0

High Frequency Clock Generator

(HFCG)

The HFCG generates the high-frequency clock based on

the system’s 32 KHz clock signal. It is controlled by the

PMC.

7.1

FEATURES

• Programmable frequency multiplier for a wide range of

output frequencies

• On power-up and WATCHDOG reset, 4 MHz default fre-

quency is set

7.2

FUNCTIONAL DESCRIPTION

The HFCG programmable frequency multiplier creates a

high-frequency output clock from a 32.768 KHz input clock.

A 5-bit and 14-bit variable define the output clock frequency.

The software changes the generated frequency by writing

new values to a buffer and enabling the frequency setting.

Either a normal or fast clock setting may be used. During a

frequency change, the CLK output is low, to prevent the

system from using a non-stable clock.

The HFCG is designed to be tightly coupled with the PMC.

The HFCG Enable signal coming from the PMC is input to

the HFCG, enabling or disabling clock generation in Idle

mode. The PMC enables the CLK for the PC87570 in Active

mode.

Figure 7-1 shows the HFCG blocks, and the operating en-

vironment.

7.2.1

Setting Clock Frequency

To change the HFCG frequency, load the HFCGN and

HFCGM variables with new values. The HFCGM variable is

loaded in two parts by writing to the HFCGML and HFCG-

MH registers.

Load the new setting (HFCGN and HFCGM values, simul-

taneously) into the frequency multiplier. The core writes the

new variables into a data input buffer. Then, a command

loads the new values into the frequency multiplier.

To set a new clock frequency:

1. Write the HFCGN value.

2. Write the HFCGML value.

3. Write the HFCGMH value.

4. Set HFCGCTRL.LOAD = 1.

When the new HFCGML, HFCGMH and HFCGN values are

loaded, the HFCG holds the output clock low until the fre-

quency multiplier locks onto the target frequency, and the

new frequency stabilizes. This automatic locking process

can take up to several milliseconds to complete.

Frequencies within the range of 4.00 to 10.00 are valid. See

Table 7-1 for a sampling of selected frequencies and their

corresponding HFCGM and HFCGN values.

Table 7-1. Frequencies of Selected Settings

1. This value is referred to as tCLKINTnom in

the AC speciﬁcations.

Control

and

Status

Data Out

Frequency

Multiplier

HFCG Enable

Data In

Peripheral Bus

CLK

32 KHz CLK

To PMC

From PMC

Figure 7-1. HFCG Schematic Diagram

Freq1 (MHz)

HFCGMH

HFCGML

HFCGN

4.00 (default)

04h

C5h

0Ah

5.00

05h

F6h

0Ah

6.00

07h

27h

0Ah

7.00

08h

58h

0Ah

8.00

09h

89h

0Ah

9.00

0Ah

BBh

0Ah

10.00

0Bh

ECh

0Ah

PC87570 Datasheet(PDF) 76 Page - National Semiconductor (TI)

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