The missing clock detect (MCD) logic detects
OSCCLK failure, using INTOSC1 as the reference clock source. This circuit only detects
complete loss of OSCCLK and does not perform any detection of frequency drift on the
OSCCLK.
This circuit monitors the OSCLK (primary clock)
using the 10MHz clock provided by the INTOSC1 (secondary clock) as a backup clock. This
circuit functions as:
- The primary clock (OSCCLK) clock keeps
ticking a 7-bit counter (named as MCDPCNT). This counter is asynchronously reset with
XRSn.
- The secondary clock (INTOSC1) clock keeps
ticking a 13-bit counter (named as MCDSCNT). This counter is asynchronously reset with
XRSn.
- Each time MCDPCNT overflows, the MCDSCNT
counter is reset. Thus, if OSCCLK is present or is not slower than INTOSC1 by a factor of
64, MCDSCNT never overflows.
- If OSCCLK stops for some reason, or is
slower than INTOSC1 by at least a factor of 64, the MCDSCNT overflows and a missing clock
condition is detected on OSCCLK.
- The above check is continuously active,
unless the MCD is disabled using MCDCR register (by making the MCLKOFF bit 1)
- If the circuit ever detects a missing
OSCCLK, the following occurs:
- The MCDSTS flag is set
- The MCDSCNT counter is frozen to
prevent further missing clock detection
- The CLOCKFAIL signal goes high, which
generates TRIP events to PWM modules and fires NMIs to CPU1.NMIWD.
- PLL is forcefully bypassed and OSCCLK
source is switched to INTOSC1 (New, System Clock Frequency = INTOSC1 Freq
10MHz)/SYSDIV). In the meantime when the clock switches to INTOSC1, the System runs on
PLL limp Clock.
- SYSPLLMULT.IMULT is zeroed out automatically in this case.
- While the MCDSTS bit is set, the
OSCCLKSRCSEL bits have no effect and OSCCLK is forcefully connected to INTOSC1.
- PLLRAWCLK going to the system is
switched to INTOSC1 automatically
- If the MCLKCLR bit is written (this is a
W=1 bit), MCDSTS bit is cleared and OSCCLK source is decided by the OSCCLKSRCSEL bits.
Writing to MCLKCLR also clears the MCDPCNT and MCDSCNT counters to allow the circuit to
re-evaluate missing clock detection. If the user wants to lock the PLL after missing clock
detection, switch the clock source to INTOSC1 (using OSCCLKSRCSEL register), do a MCLKCLR,
and re-lock the PLL.
- The MCD is enabled at power up.
Figure 3-11 shows the missing clock
logic functional flow.
Note: On a complete clock failure when OSCCLK is
dead, it can take a maximum time of 8192 INTOSC1 cycles (that is, 0.8192ms) before the
CLOCKFAIL signal goes high, after which:
- NMI is generated
- OSCCLK is switched to INTOSC1
- PWM Trip happens