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

21.3. Timer 3

Timer 3 is a 16-bit timer formed by two 8-bit SFRs: TMR3L (low byte) and TMR3H (high byte). Timer 3 may operate in 16-bit auto-reload mode, (split) 8-bit auto-reload mode, USB Start-of-Frame (SOF) capture mode, or Low-Frequency Oscillator (LFO) Rising Edge capture mode. The Timer 3 operation mode is defined by the T3SPLIT (TMR3CN.3), T3CE (TMR3CN.4) bits, and T3CSS (TMR3CN.1) bits.

Timer 3 may be clocked by the system clock, the system clock divided by 12, or the external oscillator source divided by 8. The external clock mode is ideal for real-time clock (RTC) functionality, where the internal oscillator drives the system clock while Timer 3 (and/or the PCA) is clocked by an external preci- sion oscillator. Note that the external oscillator source divided by 8 is synchronized with the system clock.

21.3.1. 16-bit Timer with Auto-Reload

When T3SPLIT (TMR3CN.3) is ‘0’ and T3CE = ‘0’, Timer 3 operates as a 16-bit timer with auto-reload. Timer 3 can be clocked by SYSCLK, SYSCLK divided by 12, or the external oscillator clock source divided by 8. As the 16-bit timer register increments and overflows from 0xFFFF to 0x0000, the 16-bit value in the Timer 3 reload registers (TMR3RLH and TM3RLL) is loaded into the Timer 3 register as shown in Figure 21.4, and the Timer 3 High Byte Overflow Flag (TMR3CN.7) is set. If Timer 3 interrupts are enabled, an interrupt will be generated on each Timer 3 overflow. Additionally, if Timer 3 interrupts are enabled and the TF3LEN bit is set (TMR3CN.5), an interrupt will be generated each time the lower 8 bits (TMR3L) over- flow from 0xFF to 0x00.

 

T3XCLK

SYSCLK / 12

 

 

 

 

0

 

External Clock / 8

 

1

 

SYSCLK

 

 

 

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

0

TR3TCLK

1

TMR3L TMR3H

TMR3RLL TMR3RLH

To ADC

 

TF3H

TMR3CN

TF3L

TF3LEN

 

T3CE

 

T3SPLIT

 

TR3

 

T3CSS

 

T3XCLK

Reload

Interrupt

Figure 21.8. Timer 3 16-Bit Mode Block Diagram

Rev. 0.5

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Silicon Laboratories C8051F346, C8051F347, C8051F341, C8051F343, C8051F340, C8051F344 Timer 3 16-Bit Mode Block Diagram