C8051F336/7/8/9
19.3. Programmable Internal Low-Frequency (L-F) Oscillator
All C8051F336/7/8/9 devices include a programmable low-frequency internal oscillator, which is calibrated
to a nominal frequency of 80 kHz. The low-frequency oscillator circuit includes a divider that can be
changed to divide the clock by 1, 2, 4, or 8, using the OSCLD bits in the OSCLCN register (see SFR Defi-
nition 19.4). Additionally, the OSCLF[3:0] bits can be used to adjust the oscillator’s output frequency.
19.3.1. Calibrating the Internal L-F Oscillator
Timers 2 and 3 include capture functions that can be used to capture the oscillator frequency, when run-
ning from a known time base. When either Timer 2 or Timer 3 is configured for L-F Oscillator Capture
Mode, a falling edge (Timer 2) or rising edge (Timer 3) of the low-frequency oscillator’s output will cause a
capture event on the corresponding timer. As a capture event occurs, the current timer value
(TMRnH:TMRnL) is copied into the timer reload registers (TMRnRLH:TMRnRLL). By recording the differ-
ence between two successive timer capture values, the low-frequency oscillator’s period can be calcu-
lated. The OSCLF bits can then be adjusted to produce the desired oscillator frequency.
SFR Definition 19.4. OSCLCN: Internal L-F Oscillator Control
Bit
7
6
5
4
3
2
1
0
Name OSCLEN OSCLRDY
OSCLF[3:0]
R.W
OSCLD[1:0]
R/W
Type
R/W
0
R
0
Reset
Varies
Varies
Varies
Varies
0
0
SFR Address = 0xE3;
Bit
Name
Function
7
OSCLEN
Internal L-F Oscillator Enable.
0: Internal L-F Oscillator Disabled.
1: Internal L-F Oscillator Enabled.
6
OSCLRDY Internal L-F Oscillator Ready.
0: Internal L-F Oscillator frequency not stabilized.
1: Internal L-F Oscillator frequency stabilized.
5:2 OSCLF[3:0] Internal L-F Oscillator Frequency Control bits.
Fine-tune control bits for the Internal L-F oscillator frequency. When set to 0000b, the
L-F oscillator operates at its fastest setting. When set to 1111b, the L-F oscillator
operates at its slowest setting.
1:0 OSCLD[1:0] Internal L-F Oscillator Divider Select.
00: Divide by 8 selected.
01: Divide by 4 selected.
10: Divide by 2 selected.
11: Divide by 1 selected.
120
Rev. 0.2
SILICON [ SILICON ]
