TMS320F28035_13 Datasheet, PDF (50/157 Pages) Texas Instruments – Piccolo Microcontrollers

TMS320F28030, TMS320F28031, TMS320F28032
TMS320F28033, TMS320F28034, TMS320F28035
SPRS584I – APRIL 2009 – REVISED JULY 2012
www.ti.com
The PLL-based clock module provides four modes of operation:
• INTOSC1 (Internal Zero-pin Oscillator 1): This is the on-chip internal oscillator 1. This can provide
the clock for the Watchdog block, core and CPU-Timer 2
• INTOSC2 (Internal Zero-pin Oscillator 2): This is the on-chip internal oscillator 2. This 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 Table 2-2 for details.
• External Clock Source Operation: If the on-chip (crystal) oscillator is not used, this mode allows it to
be bypassed. The device clocks are generated from an external clock source input on the XCLKIN pin.
Note that the XCLKIN is multiplexed with GPIO19 or GPIO38 pin. The XCLKIN input can be selected
as GPIO19 or GPIO38 via 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 3-20. Possible PLL Configuration Modes
PLL MODE
REMARKS
PLLSTS[DIVSEL]
Invoked by the user setting the PLLOFF bit in the PLLSTS register. The PLL block
is disabled in this mode. This can be useful to reduce system noise and for low
0, 1
PLL Off
power operation. The PLLCR register must first be set to 0x0000 (PLL Bypass)
2
before entering this mode. The CPU clock (CLKIN) is derived directly from the
3
input clock on either X1/X2, X1 or XCLKIN.
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 itself is bypassed but the PLL is not turned off.
0, 1
2
3
PLL Enable
Achieved by writing a non-zero 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
2
3
CLKIN AND
SYSCLKOUT
OSCCLK/4
OSCCLK/2
OSCCLK/1
OSCCLK/4
OSCCLK/2
OSCCLK/1
OSCCLK * n/4
OSCCLK * n/2
OSCCLK * n/1
3.8.4 Loss of Input Clock (NMI Watchdog Function)
The 2803x devices may be clocked from either one of the internal zero-pin oscillators
(INTOSC1/INTOSC2), the on-chip crystal oscillator, or from an external clock input. Regardless of the
clock source, in PLL-enabled and PLL-bypass mode, if the input clock to the PLL vanishes, the PLL will
issue a limp-mode clock at its output. This limp-mode clock continues to clock the CPU and peripherals at
a typical frequency of 1–5 MHz.
When the limp mode is activated, a CLOCKFAIL signal is generated that is latched as an NMI interrupt.
Depending on how the NMIRESETSEL bit has been configured, a reset to the device can be fired
immediately or the NMI watchdog counter can issue a reset when it overflows. In addition to this, the
Missing Clock Status (MCLKSTS) bit is set. The NMI interrupt could be used by the application to detect
the input clock failure and initiate necessary corrective action such as switching over to an alternative
clock source (if available) or initiate a shut-down procedure for the system.
If the software does not respond to the clock-fail condition, the NMI watchdog triggers a reset after a
preprogrammed time interval. Figure 3-13 shows the interrupt mechanisms involved.
50
Functional Overview
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TMS320F28035