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

21.2. Timer 2

Timer 2 is a 16-bit timer formed by two 8-bit SFRs: TMR2L (low byte) and TMR2H (high byte). Timer 2 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) Falling Edge capture mode. The Timer 2 operation mode is defined by the T2SPLIT (TMR2CN.3), T2CE (TMR2CN.4) bits, and T2CSS (TMR2CN.1) bits.

Timer 2 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 2 (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.

21.2.1. 16-bit Timer with Auto-Reload

When T2SPLIT = ‘0’ and T2CE = ‘0’, Timer 2 operates as a 16-bit timer with auto-reload. Timer 2 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 2 reload registers (TMR2RLH and TMR2RLL) is loaded into the Timer 2 register as shown in Figure 21.4, and the Timer 2 High Byte Overflow Flag (TMR2CN.7) is set. If Timer 2 interrupts are enabled, an interrupt will be generated on each Timer 2 overflow. Additionally, if Timer 2 interrupts are enabled and the TF2LEN bit is set (TMR2CN.5), an interrupt will be generated each time the lower 8 bits (TMR2L) overflow from 0xFF to 0x00.

 

T2XCLK

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

 

 

 

To SMBus

 

 

TL2

0

 

Overflow

TR2

TCLK

 

 

 

TMR2L TMR2H

1

 

 

TMR2RLL TMR2RLH

To ADC,

SMBus

 

TF2H

TMR2CN

TF2L

TF2LEN

 

T2CE

 

T2SPLIT

 

TR2

 

T2CSS

 

T2XCLK

Reload

Interrupt

Figure 21.4. Timer 2 16-Bit Mode Block Diagram

Rev. 0.5

251

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Silicon Laboratories C8051F343, C8051F347, C8051F346, C8051F341, C8051F340, C8051F344, C8051F345 Bit Timer with Auto-Reload