C8051F040/1/2/3/4/5/6/7

Rev. 1.5 175

14.2. External Oscillator Drive Circuit

The external oscillator circuit may drive an external crystal, ceramic resonator, capacitor, or RC network. A

CMOS clock may also provide a clock input. For a crystal or ceramic resonator configuration, the crystal/

resonator must be wired across the XTAL1 and XTAL2 pins as shown in Option 1 of Figure 14.1. In RC,

capacitor, or CMOS clock configuration, the clock source should be wired to the XTAL2 and/or XTAL1

pin(s) as shown in Option 2, 3, or 4 of Figure 14.1. The type of external oscillator must be selected in the

OSCXCN register, and the frequency control bits (XFCN) must be selected appropriately (see SFR Defini-

tion 14.4).

14.3. System Clock Selection

The CLKSL bit in register CLKSEL selects which oscillator is used as the system clock. CLKSL must be

set to ‘1’ for the system clock to run from the external oscillator; however the external oscillator may still

clock peripherals (timers, PCA) when the internal oscillator is selected as the system clock. The system

clock may be switched on-the-fly between the internal and external oscillator, so long as the selected oscil-

lator is enabled and has settled. The internal oscillator requires little start-up time and may be enabled and

selected as the system clock in the same write to OSCICN. External crystals and ceramic resonators typi-

cally require a start-up time before they are settled and ready for use as the system clock. The Crystal

Valid Flag (XTLVLD 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

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

SFR Definition 14.3. CLKSEL: Oscillator Clock Selection

Table 14.1. Internal Oscillator Electrical Characteristics

–40 to +85 °C unless otherwise specified.

Parameter Conditions Min Typ Max Units

Calibrated Internal Oscillator 

Frequency

24 24.5 25 MHz

Internal Oscillator Supply Current

(fro

)

OSCICN.7 = 1

—450— µA

External Clock Frequency

0—30MHz

XCH

(External Clock High Time)

15 — — ns

XCL

(External Clock Low Time)

15 — — ns

Bits7-1: Reserved.

Bit0: CLKSL: System Clock Source Select Bit.

0: SYSCLK derived from the Internal Oscillator, and scaled as per the IFCN bits in OSCICN.

1: SYSCLK derived from the External Oscillator circuit.

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

-------CLKSL00000000

Bit7 Bit6 Bit5 Bit4 Bit3 Bit2 Bit1 Bit0

SFR Address:

SFR Page:

0x97

C8051F040/1/2/3/4/5/6/7

176 Rev. 1.5

SFR Definition 14.4. 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 (External CMOS Clock input on XTAL1 pin).

011: External CMOS Clock Mode with divide by 2 stage (External CMOS Clock input on

XTAL1 pin).

10x: RC/C Oscillator Mode with divide by 2 stage.

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 14.1, Option 1; XOSCMD = 11x)

Choose XFCN value to match crystal frequency.

RC MODE (Circuit from Figure 14.1, Option 2; XOSCMD = 10x)

Choose XFCN value to match frequency range:

f = 1.23(10

) / (R x C), where

f = frequency of oscillation in MHz

C = capacitor value in pF

R = Pullup resistor value in k

C MODE (Circuit from Figure 14.1, Option 3; XOSCMD = 10x)

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

f = KF / (C x V

), where

f = frequency of oscillation in MHz

C = capacitor value on XTAL1, XTAL2 pins in pF

= 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:

SFR Page:

0x8C

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

C8051F040/1/2/3/4/5/6/7

Rev. 1.5 177

14.4. External Crystal Example

If a crystal or ceramic resonator is used as an external oscillator source for the MCU, the circuit should be

configured as shown in Figure 14.1, Option 1. The External Oscillator Frequency Control value (XFCN)

should be chosen from the Crystal column of the table in SFR Definition 14.4 (OSCXCN register). For

example, an 11.0592 MHz crystal requires an XFCN setting of 111b.

When the crystal oscillator is enabled, the oscillator amplitude detection circuit requires a settle time to

achieve proper bias. Introducing a delay of at least 1 ms between enabling the oscillator and checking the

XTLVLD bit will prevent a premature switch to the external oscillator as the system clock. Switching to the

external oscillator before the crystal oscillator has stabilized can result in unpredictable behavior. The rec-

ommended procedure is:

Step 1. Enable the external oscillator in crystal oscillator mode.

Step 2. Wait at least 1 ms.

Step 3. Poll for XTLVLD => '1'.

Step 4. Switch the system clock to the external oscillator.

Note: T

uning-fork crystals may require additional settling time before XTLVLD returns a valid result.

The capacitors shown in the external crystal configuration provide the load capacitance required by the

crystal for correct oscillation. These capacitors are "in series" as seen by the crystal and "in parallel" with

the stray capacitance of the XTAL1 and XTAL2 pins.

Note: The load capacitance depends upon the crystal and the manufacturer. Please refer to the crystal

data sheet when completing these calculations.

For example, a tuning-fork crystal of 32.768 kHz with a recommended load capacitance of 12.5 pF should

use the configuration shown in Figure 14.1, Option 1. The total value of the capacitors and the stray capac-

itance of the XTAL pins should equal 25 pF. With a stray capacitance of 3 pF per pin, the 22 pF capacitors

yield an equivalent capacitance of 12.5 pF across the crystal, as shown in Figure 14.2.

Figure 14.2. 32.768 kHz External Crystal Example

Important Note on External Crystals: Crystal oscillator circuits are quite sensitive to PCB layout. The

crystal should be placed as close as possible to the XTAL pins on the device. The traces should be as

short as possible and shielded with ground plane from any other traces which could introduce noise or

interference.

22 pF

32.768 kHz

XTAL1

XTAL2

10 M



Part #	C8051F041-GQ
Description	MCU 8BIT CISC 64KB FLASH 3V 64TQFP - Trays
Category	IC
Availability	In Stock
Qty	2

C8051F041-GQ

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