c8051f410 Silicon Laboratories, c8051f410 Datasheet - Page 241

• Manufacturer Part Number: c8051f410
• Description: 2.0 V, 32/16 Kb Flash, Smartclock, 12-bit Adc
• Manufacturer: Silicon Laboratories

24.2.3. External/smaRTClock Capture Mode
Capture Mode allows either the external oscillator or the smaRTClock clock to be measured against the
system clock. The external oscillator and smaRTClock clock can also be compared against each other.
Timer 2 can be clocked from the system clock, the system clock divided by 12, the external oscillator
divided by 8, or the smaRTClock clock divided by 8, depending on the T2ML (CKCON.4), T2XCLK, and
T2RCLK settings. The timer will capture either every 8 external clock cycles or every 8 smaRTClock clock
cycles, depending on the T2RCLK setting. When a capture event is generated, the contents of Timer 2
(TMR2H:TMR2L) are loaded into the Timer 2 reload registers (TMR2RLH:TMR2RLL) and the TF2H flag is
set. By recording the difference between two successive timer capture values, the external oscillator or
smaRTClock clock 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. Timer 2 should be in 16-bit auto-reload mode
when using Capture Mode.
For example, if T2ML = 1b, T2RCLK = 0b, and TF2CEN = 1b, Timer 2 will clock every SYSCLK and cap-
ture every smaRTClock clock divided by 8. If the SYSCLK is 24.5 MHz and the difference between two
successive captures is 5984, then the smaRTClock clock is:
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
external oscillator frequency when an RC network or capacitor is used to generate the signal.
External Osc. / 8
smaRTClock / 8
T2RCLK
1
0
SYSCLK / 12
Figure 24.6. Timer 2 Capture Mode Block Diagram
T2XCLK
1
0
SYSCLK
External Osc. / 8
smaRTClock / 8
M
H
T
3
M
T
3
L
CKCON
M
T
2
H
TF2CEN
M
T
2
L
1
0
M
T
1
M
T
0
S
C
A
1
S
C
A
0
TR2
Rev. 1.0
T2RCLK
1
0
Capture
TCLK
TMR2RLH
TMR2H
TMR2RLL
TMR2L
C8051F410/1/2/3
TF2CEN
T2RCLK
TF2LEN
T2XCLK
TF2H
TF2L
TR2
Interrupt
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