DSP56F826PB Просмотр технического описания (PDF) - Motorola => Freescale

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DSP56F826PB

16-bit Hybrid Controller

Motorola => Freescale 

Freescale Semiconductor, Inc.

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 5. 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 5.

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.

3.3V

Supply Regulator

2.5V

Regulator

VDDIO, VDDA

VDD

Figure 5. Example Circuit to Control Supply Sequencing

3.4 AC Electrical Characteristics

Timing waveforms in Section 3.4 are tested using the VIL and VIH levels specified in the DC Characteristics

table. The levels of VIH and VIL for an input signal are shown in Figure 6.

Input Signal

VIH

Midpoint1

Fall Time

Pulse Width

Low

High

VIL

Rise Time

Note: The midpoint is VIL + (VIH – VIL)/2.

Figure 6. Input Signal Measurement References

90%

50%

10%

Figure 7 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

20

56F826 Technical Data

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