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SPRS797F November 2012 – September 2021 TMS320F28050 , TMS320F28051 , TMS320F28052 , TMS320F28052F , TMS320F28052M , TMS320F28053 , TMS320F28054 , TMS320F28054F , TMS320F28054M , TMS320F28055

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PN|80
- MTQF010B

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8.6.3 PLL-Based Clock Module

The devices have an on-chip, PLL-based clock module. This module provides all the necessary clocking signals for the device, as well as control for low-power mode entry. The PLL has a 4-bit ratio control PLLCR[DIV] to select different CPU clock rates. The watchdog module should be disabled before writing to the PLLCR register. The watchdog module can be re-enabled (if needed) after the PLL module has stabilized, which takes 1 ms. The input clock and PLLCR[DIV] bits should be chosen in such a way that the output frequency of the PLL (VCOCLK) is at least 50 MHz.

Table 8-19 PLL Settings

PLLCR[DIV] VALUE⁽²⁾⁽³⁾	SYSCLKOUT (CLKIN)
PLLCR[DIV] VALUE⁽²⁾⁽³⁾	PLLSTS[DIVSEL] = 0 or 1⁽¹⁾	PLLSTS[DIVSEL] = 2	PLLSTS[DIVSEL] = 3
0000 (PLL bypass)	OSCCLK/4 (Default)⁽²⁾	OSCCLK/2	OSCCLK
0001	(OSCCLK × 1)/4	(OSCCLK × 1)/2	(OSCCLK × 1)/1
0010	(OSCCLK × 2)/4	(OSCCLK × 2)/2	(OSCCLK × 2)/1
0011	(OSCCLK × 3)/4	(OSCCLK × 3)/2	(OSCCLK × 3)/1
0100	(OSCCLK × 4)/4	(OSCCLK × 4)/2	(OSCCLK × 4)/1
0101	(OSCCLK × 5)/4	(OSCCLK × 5)/2	(OSCCLK × 5)/1
0110	(OSCCLK × 6)/4	(OSCCLK × 6)/2	(OSCCLK × 6)/1
0111	(OSCCLK × 7)/4	(OSCCLK × 7)/2	(OSCCLK × 7)/1
1000	(OSCCLK × 8)/4	(OSCCLK × 8)/2	(OSCCLK × 8)/1
1001	(OSCCLK × 9)/4	(OSCCLK × 9)/2	(OSCCLK × 9)/1
1010	(OSCCLK × 10)/4	(OSCCLK × 10)/2	(OSCCLK × 10)/1
1011	(OSCCLK × 11)/4	(OSCCLK × 11)/2	(OSCCLK × 11)/1
1100	(OSCCLK × 12)/4	(OSCCLK × 12)/2	(OSCCLK × 12)/1

(1) By default, PLLSTS[DIVSEL] is configured for /4. (The boot ROM changes the PLLSTS[DIVSEL] configuration to /1.) PLLSTS[DIVSEL] must be 0 before writing to the PLLCR and should be changed only after PLLSTS[PLLLOCKS] = 1.

(2) The PLL control register (PLLCR) and PLL Status Register (PLLSTS) are reset to their default state by the XRS signal or a watchdog reset only. A reset issued by the debugger or the missing clock detect logic has no effect.

(3) This register is EALLOW protected. For more information, see the System Control and Interrupts chapter of the TMS320x2805x Real-Time Microcontrollers Technical Reference Manual.

Table 8-20 CLKIN Divide Options

PLLSTS [DIVSEL]	CLKIN DIVIDE
0	/4
1	/4
2	/2
3	/1

The PLL-based clock module provides four modes of operation:

INTOSC1 (Internal Zero-pin Oscillator 1): INTOSC1 is the on-chip internal oscillator 1. INTOSC1 can provide the clock for the Watchdog block, core and CPU-Timer 2.
INTOSC2 (Internal Zero-pin Oscillator 2): INTOSC2 is the on-chip internal oscillator 2. INTOSC2 can provide the clock for the Watchdog block, core and CPU-Timer 2. Both INTOSC1 and INTOSC2 can be independently chosen for the Watchdog block, core and CPU-Timer 2.
Crystal/Resonator Operation: The on-chip (crystal) oscillator enables the use of an external crystal/resonator attached to the device to provide the time base. The crystal/resonator is connected to the X1/X2 pins. Some devices may not have the X1/X2 pins. See Section 6.2.1 for details.
External Clock Source Operation: If the on-chip (crystal) oscillator is not used, this mode allows the on-chip (crystal) oscillator to be bypassed. The device clocks are generated from an external clock source input on the XCLKIN pin. The XCLKIN is multiplexed with GPIO19 or GPIO38 pin. The XCLKIN input can be selected as GPIO19 or GPIO38 through the XCLKINSEL bit in XCLK register. The CLKCTL[XCLKINOFF] bit disables this clock input (forced low). If the clock source is not used or the respective pins are used as GPIOs, the user should disable at boot time.

Before changing clock sources, ensure that the target clock is present. If a clock is not present, then that clock source must be disabled (using the CLKCTL register) before switching clocks.

Table 8-21 Possible PLL Configuration Modes

PLL MODE	REMARKS	PLLSTS[DIVSEL]	CLKIN AND SYSCLKOUT
PLL Off	Invoked by the user setting the PLLOFF bit in the PLLSTS register. The PLL block is disabled in this mode. The PLL block being disabled can be useful in reducing system noise and for low-power operation. The PLLCR register must first be set to 0x0000 (PLL Bypass) before entering this mode. The CPU clock (CLKIN) is derived directly from the input clock on either X1/X2, X1 or XCLKIN.	0, 1	OSCCLK/4
		2	OSCCLK/2
		3	OSCCLK/1
PLL Bypass	PLL Bypass is the default PLL configuration upon power-up or after an external reset ( XRS). This mode is selected when the PLLCR register is set to 0x0000 or while the PLL locks to a new frequency after the PLLCR register has been modified. In this mode, the PLL is bypassed but is not turned off.	0, 1	OSCCLK/4
		2	OSCCLK/2
		3	OSCCLK/1
PLL Enable	Achieved by writing a nonzero value n into the PLLCR register. Upon writing to the PLLCR the device will switch to PLL Bypass mode until the PLL locks.	0, 1	OSCCLK × n/4
		2	OSCCLK × n/2
		3	OSCCLK × n/1