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.
CKCON
T T T T T T S S
3 3 2 2 1 0 C C
MM M MM M A A
T3XCLK[1:0]
H L H L
1 0
SYSCLK / 12
00
01
0
1
TCLK
TR3
TMR3L
TMR3H
External Clock / 8
SYSCLK
Capture
TF3CEN
Low-Frequency
Oscillator
TF3H
TF3L
TF3LEN
TF3CEN
T3SPLIT
TR3
Interrupt
TMR3RLL TMR3RLH
T3XCLK1
T3XCLK0
Figure 24.9. Timer 3 Low-Frequency Oscillation Capture Mode Block Diagram
Rev. 0.2
205
SILICON [ SILICON ]
