C8051F321-GMR Datasheet, PDF (117/250 Pages) Silicon Laboratories – Full Speed USB, 16 k ISP FLASH MCU Family

C8051F320/1
Equation 13.1. Typical Change in Internal Oscillator Period with OSCICL
T  0.0025  ------1------  OSCICL
f_BASE
On C8051F320/1 devices, OSCICL is factory calibrated to obtain a 12 MHz base frequency (f_BASE). Sec-
tion 13.1.1 details oscillator programming for C8051F320/1 devices. Electrical specifications for the preci-
sion internal oscillator are given in Table 13.3 on page 125. Note that the system clock may be derived
from the programmed internal oscillator divided by 1, 2, 4, or 8, as defined by the IFCN bits in register
OSCICN. The divide value defaults to 8 following a reset.
13.1.1. Programming the Internal Oscillator on C8051F320/1 Devices
The OSCICL reset value is factory calibrated to result in a 12 MHz internal oscillator with a ±1.5% accu-
racy; this frequency is suitable for use as the USB clock (see Section 13.4). Software may modify the fre-
quency of the internal oscillator as described below.
Important Note: Once the internal oscillator frequency has been modified, the internal oscillator may not
be used as the USB clock as described in Section 13.4. The internal oscillator frequency will reset to its
original factory-calibrated frequency following any device reset, at which point the oscillator is suitable for
use as the USB clock.
Software should read and adjust the value of OSCICL according to Equation 13.1 to obtain the desired fre-
quency. The example below shows how to obtain an 11.6 MHz internal oscillator frequency.
f_BASE is the internal oscillator reset frequency; T_BASE is the reset oscillator period.
f_DES is the desired internal oscillator frequency; T_DES is the desired oscillator period.
f_BASE = 12000000Hz
T_BASE
=
-----------1------------
12000000
s
f_DES = 11600000Hz
T_DES
=
-----------1------------
11600000
s
The required change in period (T_DES) is the difference between the base period and the desired period.
T_DES
=
-----------1------------
11600000
–
-----------1------------
12000000
=
2.87  10^–9s
Using Equation 13.1 and the above calculations, find OSCICL:
2.87  10^–9 = 0.0025  ------1------  OSCICL
f_BASE
OSCICL = 13.79
OSCICL is rounded to the nearest integer (14) and added to the reset value of register OSCICL.
Rev. 1.4
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