C8051T323-GMR Silicon Labs, C8051T323-GMR Datasheet - Page 131
C8051T323-GMR
Manufacturer Part Number
C8051T323-GMR
Description
8-bit Microcontrollers - MCU USB-OTP-16K-QFN28
Manufacturer
Silicon Labs
Datasheet
1.C8051T322-GQR.pdf
(292 pages)
Specifications of C8051T323-GMR
Rohs
yes
Core
8051
Data Bus Width
8 bit
Processor Series
C8051
- Current page: 131 of 292
- Download datasheet (2Mb)
C8051T620/621/320/321/322/323
21.6.2. External RC Example
If an RC network is used as an external oscillator source for the MCU, the circuit should be configured as
shown in Figure 21.1, “RC Mode”. The capacitor should be no greater than 100 pF; however, for very small
capacitors, the total capacitance may be dominated by parasitic capacitance in the PCB layout. To deter-
mine 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, according to Equation , where
f = the frequency of oscillation in MHz, C = the capacitor value in pF, and R = the pull-up resistor value in
k.
3
f
=
1.23 10
R C
Equation 21.1. RC Mode Oscillator Frequency
For example: If the frequency desired is 100 kHz, let R = 246 k and C = 50 pF:
3
3
f = 1.23( 10
) / RC = 1.23 ( 10
) / [ 246 x 50 ] = 0.1 MHz = 100 kHz
Referring to the table in SFR Definition 21.6, the required XFCN setting is 010b.
21.6.3. External Capacitor Example
If a capacitor is used as an external oscillator for the MCU, the circuit should be configured as shown in
Figure 21.1, “C Mode”. The capacitor should be no greater than 100 pF; however, for very small capaci-
tors, the total capacitance may be dominated by parasitic capacitance in the PCB layout. To determine the
required External Oscillator Frequency Control value (XFCN) in the OSCXCN Register, select the capaci-
tor to be used and find the frequency of oscillation according to Equation , where f = the frequency of oscil-
lation in MHz, C = the capacitor value in pF, and V
= the MCU power supply in Volts.
DD
f
=
KF
C V
DD
Equation 21.2. C Mode Oscillator Frequency
For example: Assume V
= 3.0 V and f = 150 kHz:
DD
f = KF / (C x VDD)
0.150 MHz = KF / (C x 3.0)
Since the frequency of roughly 150 kHz is desired, select the K Factor from the table in SFR Definition 21.6
(OSCXCN) as KF = 22:
0.150 MHz = 22 / (C x 3.0)
C x 3.0 = 22 / 0.150 MHz
C = 146.6 / 3.0 pF = 48.8 pF
Therefore, the XFCN value to use in this example is 011b and C = 50 pF.
Rev. 1.1
131
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