Figure 6-24 shows the recommended oscillator connections when WKUP_OSC0_XI is connected to a 1.8V
LVCMOS square-wave digital clock source.
Note:
- A DC steady-state condition is not
allowed on WKUP_OSC0_XI when the oscillator is powered up. This is not allowed because
WKUP_OSC0_XI is internally AC coupled to a comparator that can enter an unknown state
when DC is applied to the input. Therefore, application software must power down
WKUP_OSC0 any time WKUP_OSC0_XI is not toggling between logic states.
- The LVCMOS clock signal sourcing the
WKUP_OSC0_XI input must have monotonic transitions. The clock source should be connected
to WKUP_OSC0_XI with a point-to-point connection, via a series termination resistor
placed near the clock source. The series termination resistor value should match the
clock source output impedance to the transmission line impedance. For example, the
series termination resistor value needs to be 20 ohms if the clock source has an output
impedance of 30 ohms and the PCB signal trace has a characteristic impedance of 50 ohms.
This allows the reflection that returns from the far end of the un-terminated
transmission line to be completely absorbed such that is does not introduce any
non-monotonic events on the signal.
- The PCB trace length connecting the LVCMOS clock source to WKUP_OSC0_XI should be
minimized. This reduces capacitive loading and decreases probability of external noise
sources coupling into the clock signal. Reduced capacitive loading improves rise/fall
times of the clock signal which reduces the probability of jitter being introduced in
the system.
Table 6-20 WKUP_OSC0 LVCMOS Digital Clock Source
Requirements
| PARAMETER |
MIN |
TYP |
MAX |
UNIT |
| Fxtal |
Frequency |
|
25 |
|
MHz |
| Frequency Stability and Tolerance |
Ethernet
RGMII and RMII not used |
|
|
±100 |
ppm |
| Ethernet
RGMII and RMII using derived clock |
|
|
±50 |
| DC |
Duty Cycle |
45 |
|
55 |
% |
| tR/F |
Rise/Fall Time (10%-90% rise, 90%-10% fall) |
|
|
4(1) |
ns |
| JPeriod(RMS) |
Period Jitter, RMS (100k samples) |
|
|
20 |
ps |
| JPeriod(PK-PK) |
Period Jitter, Peak to Peak (100k samples) |
|
|
300 |
ps |
| JPhase(RMS) |
Phase Jitter, RMS (BW 100Hz to 1MHz) |
|
|
10(2) |
ps |
(1) Most LVCMOS oscillator datasheets define
their maximum Output Rise/Fall times with a capacitive load much larger than the actual
load that will be applied by the combined PCB trace capacitance and WKUP_OSC0_XI input
capacitance. It should not be difficult to find a LVCMOS oscillator that meets this
requirement. However, the system designer must confirm the LVCMOS oscillator selected will
provide the appropriate rise/fall time to WKUP_OSC0_XI input.
(2) Most LVCMOS oscillator datasheets define
their max RMS Phase Jitter using a larger bandwidth integration range than required by
this device. To get a more appropriate value, it may be necessary to contact the LVCMOS
oscillator manufacture and ask them to provide a maximum RMS Phase Jitter using the same
bandwidth integration range that has been defined for this parameter.