C8051F007 View Datasheet(PDF) - Silicon Laboratories

Part Name

Description

MFG CO.

C8051F007

Mixed-Signal 32KB ISP FLASH MCU Family

Silicon Laboratories 

C8051F000/1/2/5/6/7

C8051F010/1/2/5/6/7

14.1. External Crystal Example

If a crystal or ceramic resonator were used to generate the system clock for the MCU, the circuit would be as shown

in Figure 14.1, Option 1. For an ECS-110.5-20-4 crystal, the resonate frequency is 11.0592MHz, the intrinsic

capacitance is 7pF, and the ESR is 60Ω. The compensation capacitors should be 33pF each, and the PWB parasitic

capacitance is estimated to be 2pF. The appropriate External Oscillator Frequency Control value (XFCN) from the

Crystal column in the table in Figure 14.3 (OSCXCN Register) should be 111b.

Because the oscillator detect circuitry needs time to settle after the crystal oscillator is enabled, software should wait

at least 1ms between enabling the crystal oscillator and polling the XTLVLD bit. The recommend procedure is:

1. Enable the external oscillator

2. Wait at least 1 ms

3. Poll for XTLVLD '0' ==> '1'

4. Switch to the external oscillator

Switching to the external oscillator before the crystal oscillator has stabilized could result in unpredictable behavior.

NOTE: 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, keeping the traces as short as possible and shielded with ground plane from any

other traces which could introduce noise or interference.

14.2. External RC Example

If an external RC network were used to generate the system clock for the MCU, the circuit would be as shown in

Figure 14.1, Option 2. The capacitor must be no greater than 100pF, but using a very small capacitor will increase

the frequency drift due to the PWB parasitic capacitance. To determine the required External Oscillator Frequency

Control value (XFCN) in the OSCXCN Register, first select the RC network value to produce the desired frequency

of oscillation. If the frequency desired is 100kHz, let R = 246kΩ and C = 50pF:

f = 1.23(103)/RC = 1.23(103) / [246 * 50] = 0.1MHz = 100kHz

XFCN ≥ log2(f/25kHz)

XFCN ≥ log2(100kHz/25kHz) = log2(4)

XFCN ≥ 2, or code 010

14.3. External Capacitor Example

If an external capacitor were used to generate the system clock for the MCU, the circuit would be as shown in

Figure 14.1, Option 3. The capacitor must be no greater than 100pF, but using a very small capacitor will increase

the frequency inaccuracy due to the PWB parasitic capacitance. To determine the required External Oscillator

Frequency Control value (XFCN) in the OSCXCN Register, select the capacitor to be used and find the frequency

of oscillation from the equations below. Assume AV+ = 3.0V and C = 50pF:

f = KF / (C * VDD) = KF / (50 * 3)

f = KF / 150

If a frequency of roughly 90kHz is desired, select the K Factor from the table in Figure 14.3 as KF = 13:

f = 13 /150 = 0.087MHz, or 87kHz

Therefore, the XFCN value to use in this example is 011.

Rev. 1.7

102

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