
Functional Description and Application Information
S12S Clocks and Reset Generator (S12SCRGV1)
MM912F634
Freescale Semiconductor
254
4.32.5.2
Description of Reset Operation
NOTE
External circuitry connected to the RESET pin should not include a large capacitance that
would interfere with the ability of this signal to rise to a valid logic one within 256 DCO Clock
cycles after the low drive is released.
The reset sequence is initiated by any of the following events:
Low level is detected at the RESET pin (External Reset).
Power-on is detected.
Illegal Address Access is detected (see MMC Block Guide for details).
COP watchdog times out.
Oscillator monitor failure is detected.
Upon detection of any reset event, an internal circuit drives the RESET pin low for 516 DCO Clock cycles. Depending on internal
synchronization latency, it can also be 517 DCO Clock cycles (see
Figure 77). Since entry into reset is asynchronous, it does not
require a running DCO Clock. However, the internal reset circuit of the 9S12I32PIMV1 cannot sequence out of current reset
condition without a running DCO Clock. After 516 DCO Clock cycles, the RESET pin is released. The reset generator of the
9S12I32PIMV1 waits for additional 256 DCO Clock cycles and then samples the RESET pin to determine the originating source.
Table 334 shows which vector will be fetched.
The internal reset of the MCU remains asserted while the reset generator completes the 768 DCO Clock long reset sequence.
In case the RESET pin is externally driven low for more than these 768 DCO Clock cycles (External Reset), the internal reset
remains asserted longer.
Figure 77. RESET Timing
4.32.5.2.1
Oscillator Monitor Reset
In case of loss of clock, or the oscillator frequency is below the failure assert frequency fOMFA (see device electrical characteristics
for values), the 9S12I32PIMV1 generates a Oscillator Monitor Reset.
Table 334. Reset Vector Selection
Sampled RESET Pin
(256 cycles after release)
Oscillator monitor
fail pending
COP timeout
pending
Vector Fetch
1
0
POR /Illegal Address Access/External pin RESET
1
X
Oscillator Monitor Fail
1
0
1
COP time out
0
X
POR /Illegal Address Access/ External pin RESET
) (
)
(
)
DCO Clock
516 cycles
256 cycles
S12SCRG drives RESET pin low
possibly DCO Clock n
ot running
possibly RESET driv-
en low externally
)
(
RESET
RESET pin
released