TMS320VC549 datasheet(34/118 Pages) TI1

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TMS320C54x, TMS320LC54x, TMS320VC54x

FIXED-POINT DIGITAL SIGNAL PROCESSORS

SPRS039C – FEBRUARY 1996 – REVISED DECEMBER 1999

POST OFFICE BOX 1443

• HOUSTON, TEXAS 77251–1443

software-programmable PLL (’545A, ’546A, ’548, and ’549) (continued)

Immediately following reset, the clock mode is determined by the values of the three external pins: CLKMD1,

CLKMD2, and CLKMD3. The modes corresponding to the CLKMD pins are shown in Table 6.

Table 6. Clock Mode Settings at Reset

CLKMD1

CLKMD2

CLKMD3

CLKMD REGISTER

RESET VALUE

CLOCK MODE

0000h

Divide-by-two, with external source

1000h

Divide-by-two, with external source

2000h

Divide-by-two, with external source

4000h

Divide-by-two, internal oscillator enabled

6000h

Divide-by-two, with external source

7000h

Divide-by-two, internal oscillator enabled†

0007h

PLL

× 1 with external source

—

Stop mode

† Reserved mode (’549 only). Do not use in normal operation.

Following reset, the software-programmable PLL can be programmed to any configuration desired, as

described above. Note that when the PLL

× 1 with external source option (CLKMD[1–3]=101) is selected during

reset, the internal PLL lock-count timer is not active; therefore, the system must delay releasing reset in order

to allow for the PLL lock-time delay. Also, note that both the hardware- and the software-programmable PLLs

require the device to be reset after power up to begin functioning properly.

programming considerations when using the software-programmable PLL

The software-programmable PLL offers many different options in startup configurations, operating modes, and

power-saving features. Programming considerations and several software examples are presented here to

illustrate the proper use of the software-programmable PLL at start-up, when switching between different

clocking modes, and before and after IDLE1/IDLE2/IDLE3 instruction execution.

use of the PLLCOUNT programmable lock timer

During the lockup period, the PLL should not be used to clock the ’54x. The PLLCOUNT programmable lock

timer provides a convenient method of automatically delaying clocking of the device by the PLL until lock is

achieved.

The PLL lock timer is a counter, loaded from the PLLCOUNT field in the CLKMD register, that decrements from

its preset value to 0. The timer can be preset to any value from 0 to 255, and its input clock is CLKIN divided

by 16. The resulting lockup delay can therefore be set from 0 to 255

16 CLKIN cycles.

The lock timer is activated when the clock generator operating mode is switched from DIV to PLL (see the

section describing switching from DIV mode to PLL mode). During the lockup period, the clock generator

continues to operate in DIV mode; after the PLL lock timer has decremented to zero, the PLL begins clocking

the ’54x.

Accordingly, the value loaded into PLLCOUNT is chosen based on the following relationship:

PLLCOUNT > Lockup Time / (16

tCLKIN)

where tCLKIN is the input reference clock period and lockup time is the required PLL lockup time as shown in

Figure 8.

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