C8051F340/1/2/3/4/5/6/7

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

T

T

T

T

T

T

T

T

F

F

F

3

3

R

3

3

3

3

3

C

S

3

C

X

H

L

L

E

P

 

S

C

 

 

E

 

L

 

S

L

 

 

N

 

I

 

 

K

 

 

 

 

T

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CKCON

T T T T T T S S 3 3 2 2 1 0 C C M M M M M M A A

H L H L 1 0

SYSCLK / 12

0

 

 

 

 

 

0

TR3

TCLK

 

 

External Clock / 8

1

TMR3L

TMR3H

 

 

 

 

 

 

SYSCLK

1

 

 

 

 

USB Start-of-Frame (SOF)

0

 

 

 

 

 

 

Capture

TMR3RLL

TMR3RLH

 

 

 

 

Low-Frequency Oscillator

1

 

 

 

 

Falling Edge

 

 

 

 

 

 

 

 

 

 

T3CSS

Enable

Interrupt

 

 

 

 

 

To ADC

Figure 21.10. Timer 3 Capture Mode (T3SPLIT = ‘0’)

Rev. 0.5

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Image 259
Silicon Laboratories C8051F343, C8051F347, C8051F346 Timer 3 Capture Mode T3SPLIT = ‘0’, USB Start-of-Frame Capture