Rev. 1.4 121

C8051F320/1

SFR Definition 13.3. OSCXCN: External Oscillator Control

Bit7: XTLVLD: Crystal Oscillator Valid Flag.

(Read only when XOSCMD = 11x.)

0: Crystal Oscillator is unused or not yet stable.

1: Crystal Oscillator is running and stable.

Bits6–4: XOSCMD2–0: External Oscillator Mode Bits.

00x: External Oscillator circuit off.

010: External CMOS Clock Mode.

011: External CMOS Clock Mode with divide by 2 stage.

100: RC Oscillator Mode.

101: Capacitor Oscillator Mode.

110: Crystal Oscillator Mode.

111: Crystal Oscillator Mode with divide by 2 stage.

Bit3: RESERVED. Read = 0, Write = don't care.

Bits2–0: XFCN2–0: External Oscillator Frequency Control Bits.

000–111: See table below:

CRYSTAL MODE (Circuit from Figure 13.1, Option 1; XOSCMD = 11x)

Choose XFCN value to match crystal or resonator frequency.

RC MODE (Circuit from Figure 13.1, Option 2; XOSCMD = 10x)

Choose XFCN value to match frequency range:

f = 1.23(10

) / (R * C), where

f = frequency of clock in MHz

C = capacitor value in pF

R = Pull-up resistor value in k

C MODE (Circuit from Figure 13.1, Option 3; XOSCMD = 10x)

Choose K Factor (KF) for the oscillation frequency desired:

f = KF / (C * VDD), where

f = frequency of clock in MHz

C = capacitor value the XTAL2 pin in pF

VDD = Power Supply on MCU in volts

R R/W R/W R/W R R/W R/W R/W Reset Value

XTLVLD XOSCMD2 XOSCMD1 XOSCMD0 - XFCN2 XFCN1 XFCN0 00000000

Bit7 Bit6 Bit5 Bit4 Bit3 Bit2 Bit1 Bit0 SFR Address:

0xB1

XFCN Crystal (XOSCMD = 11x) RC (XOSCMD = 10x) C (XOSCMD = 10x)

000 f  32 kHz f 25 kHz K Factor = 0.87

001 32 kHz f 84 kHz 25 kHz f 50 kHz K Factor = 2.6

010 84 kHz  f 225 kHz 50 kHz f 100 kHz K Factor = 7.7

011 225 kHz  f 590 kHz 100 kHz f 200 kHz K Factor = 22

100 590 kHz  f 1.5 MHz 200 kHz f 400 kHz K Factor = 65

101 1.5 MHz  f 4 MHz 400 kHz f 800 kHz K Factor = 180

110 4 MHz  f 10 MHz 800 kHz f 1.6 MHz K Factor = 664

111 10 MHz  f 30 MHz 1.6 MHz f 3.2 MHz K Factor = 1590

C8051F320/1

122 Rev. 1.4

13.3. 4x Clock Multiplier

The 4x Clock Multiplier allows a 12 MHz oscillator to generate the 48 MHz clock required for Full Speed

USB communication (see Section “15.4. USB Clock Configuration” on page 146). A divided version of the

Multiplier output can also be used as the sy

stem clock. See Section 13.4 for details on system clock and

USB clock source selection.

The 4x Clock Multiplier is configured via the CLKMUL register. The procedure for configuring and enabling

the 4x

Clock Multiplier is as follows:

1. Reset the Multiplier by writin

g 0x00 to register CLKMUL.

2. Select the Multiplier input source via the MULSEL bits.

3. Enable the Multiplier with the MULEN bit (CLKMUL | = 0x80).

4. Delay for >5 µs.

5. Initialize the Multiplier w

ith the MULINIT bit (CLKMUL | = 0xC0).

6. Poll for MULRDY => ‘1’.

Important Note: When using an external oscillator as the input to the 4x Clock Multiplier, the exter-

nal source must be enabled and stable befo

re the Multiplier is initialized. See Section 13.4 for

details on selecting an external oscillator source.

SFR Definition 13.4. CLKMUL: Clock Multiplier Control

Bit7: MULEN: Clock Multiplier Enable

0: Clock Multiplier disabled.

1: Clock Multiplier enabled.

Bit6: MULINIT: Clock Multiplier Initialize

This bit should be a ‘0’ when the Clock Multiplier is enabled. Once enabled, writing a ‘1’ to

this bit will initialize the Clock Multiplier. The MULRDY bit reads ‘1’ when the Clock Multiplier

is stabilized.

Bit5: MULRDY: Clock Multiplier Ready

This read-only bit indicates the status of the Clock Multiplier.

0: Clock Multiplier not ready.

1: Clock Multiplier ready (locked).

Bits4–2: Unused. Read = 000b; Write = don’t care.

Bits1–0: MULSEL: Clock Multiplier Input Select

These bits select the clock supplied to the Clock Multiplier.

R/W R/W R R/W R/W R/W R/W R/W Reset Value

MULEN MULINIT MULRDY - - - MULSEL 00000000

Bit7 Bit6 Bit5 Bit4 Bit3 Bit2 Bit1 Bit0 SFR Address

0xB9

MULSEL Selected Clock

00 Internal Oscillator

01 External Oscillator

10 External Oscillator / 2

11 RESERVED

Rev. 1.4 123

C8051F320/1

13.4. System and USB Clock Selection

The internal oscillator requires little start-up time and may be selected as the system or USB clock immedi-

ately following the OSCICN write

that enables the internal oscillator. External crystals and ceramic resona-

tors typically require a start-up time before they are settled and ready for use. The Crystal Valid Flag

(XTL

VLD in register OSCXCN) is set to ‘1’ by hardware when the external oscillator is settled. To avoid

reading a false XTLVLD, in crystal mode software should delay at least 1 ms between enabling the

xternal oscillator and checking XTLVLD. RC and C modes typically require no startup time.

13.4.1. System Clock Selection

The CLKSL[1:0] bits in register CLKSEL select which oscillator source is used as the system clock.

CLKSL[1:0] must be set to 01b for the system clock to run from the external oscillator; however the exter-

nal oscillator may still clock certain peripherals (timers, PCA

, USB) when the internal oscillator is selected

as the system clock. The system clock may be switched on-the-fly between the internal oscillator, external

oscillator, and 4x Clock Multiplier so long as the selected oscillator is enabled and has settled.

13.4.2. USB Clock Selection

The USBCLK[2:0] bits in register CLKSEL select which oscillator source is used as the USB clock. The

USB clock may be derived from the 4x Clock Multiplier output, a divided version of the internal oscillator, or

a divided version of the external oscillator. Note that the USB clock must be 48 MHz when operating USB0

a Full Speed Function; the USB clock must be 6 MHz when operating USB0 as a Low Speed Function.

See Figure 13.5 for USB clock selection options.

Some example USB clock configurations for Full and Low Speed mode are given below:

Table 13.1. Typical USB Full Speed Clock Settings

Internal Oscillator

Clock Signal Input Source Selection Register Bit Settings

USB Clock Clock Multiplier USBCLK = 000b

Clock Multiplier Input Internal Oscillator* MULSEL = 00b

Internal Oscillator Divide by 1 IFCN = 11b

External Oscillator

Clock Signal Input Source Selection Register Bit Settings

USB Clock Clock Multiplier USBCLK = 000b

Clock Multiplier Input External Oscillator MULSEL = 01b

External Oscillator

Crystal Oscillator Mode

12 MHz Crystal

XOSCMD = 110b

XFCN = 111b

*Note: Clock Recovery must be enabled for this configuration.

Part #	C8051F321-GMR
Description	16KB,10ADC,USB,28PIN MCU (LEAD FREE) MLP28 -40 TO 85 DEG.
Category	IC
Availability	In Stock
Qty	560

Qty	Price
1 - 24	$10.41286
25 - 60	$8.28296
61 - 128	$7.80964
129 - 275	$7.25745
276 +	$6.46859

C8051F321-GMR

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Technical Document