Silicon Laboratories C8051F347 - Manual (Page 251)

Rev. 1.3 251
C8051F340/1/2/3/4/5/6/7/8/9/A/B/C/D
21.3.3. USB Start-of-Frame Capture
When T3CE = ‘1’, Timer 3 will operate in one of two special capture modes. The capture event can be
selected between a USB Start-of-Frame (SOF) capture, and a Low-Frequency Oscillator (LFO) Rising
Edge capture, using the T3CSS bit. The USB SOF capture mode can be used to calibrate the system clock
or external oscillator against the known USB host SOF clock. The LFO rising-edge capture mode can be
used to calibrate the internal Low-Frequency Oscillator against the internal High-Frequency Oscillator or
an external clock source. When T3SPLIT = ‘0’, Timer 3 counts up and overflows from 0xFFFF to 0x0000.
Each time a capture event is received, the contents of the Timer 3 registers (TMR3H:TMR3L) are latched
into the Timer 3 Reload registers (TMR3RLH:TMR3RLL). A Timer 3 interrupt is generated if enabled.
Figure 21.10. Timer 3 Capture Mode (T3SPLIT = ‘0’)
External Clock / 8
SYSCLK / 12
SYSCLK
TMR3L TMR3H
TMR3RLL TMR3RLH
TCLK
0
1
TR3
0
1
Interrupt
To ADC
CKCON
T
3
M
H
T
3
M
L
S
C
A
0
S
C
A
1
T
0
M
T
2
M
H
T
2
M
L
T
1
M
Capture
Enable
TMR3CN
T
F
3
H
T
F
3
L T
3
X
C
L
K
T
3
C
S
S
T
R
3
T
F
3
L
E
N
T
3
C
E
T
3
S
P
L
I
T
USB Start-of-Frame (SOF) 0
1
T3CSS
Low-Frequency Oscillator
Falling Edge