TMS320F28335: The time taken for boot process to be finished

The number t_h(boot-mode) provides an estimate of this value. The boot-to-flash option is the fastest boot option available, since no boot-loading through a peripheral is involved. The moment the boot-mode-select pins are read and it is determined by the boot-ROM that a branch-to-flash is to be executed, that is carried out right away. This takes about 200 t_c(OSCCLK) cycles as shown in the datasheet.

Page 46 of www.ti.com/lit/SPRS439. 

yoonseok shim said:

does 'branch to FLASH' mean that Program counter designates the start memory address where a application loaded in the FLASH is executed from at first?

Yes.

yoonseok shim said:

and Is the boot process executed under OSCCLK modified by PLL in case the PLL is initialized?

The PLL is not initialized in the boot-ROM. Please refer to 2.2.3 PLL Multiplier and DIVSEL Selection in page 173 of www.ti.com/lit/SPRUI07. 

yoonseok shim said:

I guess the OSCCLK is the one which is not modified by PLLSTS register.

OSCCLK is the input to PLL. PLLSTS is use to divide the output of the PLL. Please refer to Figure 1-19. OSC and PLL Block in page 66 of www.ti.com/lit/SPRUI07. 

Correct. OSCCLK is just the generic name for the input clock to the PLL. The source for OSCCLK can be an external oscillator, quartz crystal or a ceramic resonator.