C8051F970-A-GM Datasheet, PDF (396/454 Pages) Silicon Laboratories – Low Power Capacitive Sensing MCU with up to 32 kB of Flash

C8051F97x
32.2.3. Comparator 0/SmaRTClock Capture Mode
The Capture Mode in Timer 2 allows either Comparator 0 or the SmaRTClock period to be measured against the
system clock or the system clock divided by 12. Comparator 0 and the SmaRTClock period can also be compared
against each other. Timer 2 Capture Mode is enabled by setting TF2CINT to 1. Timer 2 should be in 16-bit auto-
reload mode when using Capture Mode.
When Capture Mode is enabled, a capture event will be generated either every Comparator 0 rising edge or every
8 SmaRTClock clock cycles, depending on the T2XCLK1 setting. When the capture event occurs, the contents of
Timer 2 (TMR2H:TMR2L) are loaded into the Timer 2 reload registers (TMR2RLH:TMR2RLL) and the TF2H flag is
set (triggering an interrupt if Timer 2 interrupts are enabled). By recording the difference between two successive
timer capture values, the Comparator 0 or SmaRTClock period can be determined with respect to the Timer 2
clock. The Timer 2 clock should be much faster than the capture clock to achieve an accurate reading.
For example, if T2ML = 1b, T2XCLK1 = 0b, and TF2CINT = 1b, Timer 2 will clock every SYSCLK and capture
every SmaRTClock clock divided by 8. If the SYSCLK is 24.5 MHz and the difference between two successive cap-
tures is 5984, then the SmaRTClock clock is as follows:
24.5 MHz/(5984/8) = 0.032754 MHz or 32.754 kHz.
This mode allows software to determine the exact SmaRTClock frequency in self-oscillate mode and the time
between consecutive Comparator 0 rising edges, which is useful for detecting changes in the capacitance of a
Touch Sense Switch.
T2XCLK[1:0]
SYSCLK / 12
CKCON
TTTTTTSS
3 3 2 2 1 0CC
X0 M M M M M M A A
HLHL 10
SmaRTClock / 8
11
SYSCLK
T2XCLK1
SmaRTClock / 8
0
0
TR2
1
TF2CEN
TCLK _{TMR2L TMR2H}
Capture
TMR2RLL TMR2RLH
TF2H
TF2L
TF2LEN
TF2CEN
T2SPLIT
TR2
T2XCLK1
T2XCLK0
Interrupt
Figure 32.6. Timer 2 Capture Mode Block Diagram
Rev 1.0
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