C8051F340-GQR Datasheet, PDF (251/276 Pages) Silicon Laboratories – Full Speed USB Flash MCU Family

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.
TMR3CN
TTTTTTTT
FFF 3 3R3 3
3 3 3CS3CX
HL LEP SC
E L SL
NI K
T
SYSCLK / 12
0
External Clock / 8
1
SYSCLK
CKCON
TTTTTTSS
3 3 2 2 1 0CC
MMMMMMA A
HLHL 10
0
TR3
1
TCLK _{TMR3L TMR3H}
USB Start-of-Frame (SOF)
Low-Frequency Oscillator
Falling Edge
0
Capture
TMR3RLL TMR3RLH
1
T3CSS
Enable
Interrupt
Figure 21.10. Timer 3 Capture Mode (T3SPLIT = ‘0’)
To ADC
Rev. 1.3
251