C8051F336_08 Datasheet, PDF (198/227 Pages) Silicon Laboratories – Mixed-Signal Byte-Programmable EPROM MCU

C8051F336/7/8/9
24.3.3. Low-Frequency Oscillator (LFO) Capture Mode
The Low-Frequency Oscillator Capture Mode allows the LFO clock to be measured against the system
clock or an external oscillator source. Timer 3 can be clocked from the system clock, the system clock
divided by 12, or the external oscillator divided by 8, depending on the T3ML (CKCON.6), and
T3XCLK[1:0] settings.
Setting TF3CEN to 1 enables the LFO Capture Mode for Timer 3. In this mode, T3SPLIT should be set to
0, as the full 16-bit timer is used. Upon a falling edge of the low-frequency oscillator, the contents of
Timer 3 (TMR3H:TMR3L) are loaded into the Timer 3 reload registers (TMR3RLH:TMR3RLL) and the
TF3H flag is set. By recording the difference between two successive timer capture values, the LFO clock
frequency can be determined with respect to the Timer 3 clock. The Timer 3 clock should be much faster
than the LFO to achieve an accurate reading. This means that the LFO/8 should not be selected as the
timer clock source in this mode.
T3XCLK[1:0]
CKCON
TTTTTTSS
3 3 2 2 1 0CC
MMMMMMA A
HLHL 10
SYSCLK / 12
00
External Clock / 8
01
SYSCLK
0
TR3
1
TCLK
TMR3L TMR3H
Capture
Low-Frequency
Oscillator
TF3CEN
TMR3RLL TMR3RLH
TF3H
TF3L
TF3LEN
TF3CEN
T3SPLIT
TR3
T3XCLK1
T3XCLK0
Interrupt
Figure 24.9. Timer 3 Low-Frequency Oscillation Capture Mode Block Diagram
198
Rev.1.0