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56F826 Technical Data, Rev. 14
24
Freescale Semiconductor
VDD should not be allowed to rise early (1). This is usually avoided by running the regulator for the VDD
supply (2.5V) from the voltage generated by the 3.3V VDDIO supply, see Figure 3-3. This keeps VDD from rising faster than VDDIO.
VDD should not rise so late that a large voltage difference is allowed between the two supplies (2).
Typically this situation is avoided by using external discrete diodes in series between supplies, as shown
in Figure 3-3. The series diodes forward bias when the difference between VDDIO and VDD reaches approximately 1.4, causing VDD to rise as VDDIO ramps up. When the VDD regulator begins proper
operation, the difference between supplies will typically be 0.8V and conduction through the diode chain
reduces to essentially leakage current. During supply sequencing, the following general relationship
should be adhered to:
VDDIO > VDD > (VDDIO - 1.4V)
In practice, VDDA is typically connected directly to VDDIO with some filtering.
Figure 3-3 Example Circuit to Control Supply Sequencing
3.4 AC Electrical Characteristics
Timing waveforms in Section 3.4 are tested using the VIL and VIHlevels specified in the DC Characteristics table. The levels of VIH and VIL for an input signal are shown in Figure 3-4. Figure 3-4 Input Signal Measurement References
Figure 3-5 shows the definitions of the following signal states:
Active state, when a bus or signal is driven, and enters a low impedance state
Tri-stated, when a bus or signal is placed in a high impedance state
Data Valid state, when a signal level has reached VOL or VOH
Data Invalid state, when a signal level is in transition between VOL and VOH
3.3V
Regulator
2.5V
Regulator
Supply
VDD
VDDIO, VDDA
VIH
VIL
Fall Time
Input Signal
Note: The midpoint is VIL + (VIH – VIL)/2.
Midpoint1
Low
High
Pulse Width
90%
50%
10%
Rise Time