DSP56857 datasheet(172/662 Pages) FREESCALE

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Block Diagram

5685X Digital Signal Controller User Manual, Rev. 4

7-4

Freescale Semiconductor

7.3 Block Diagram

Figure 7-1. POR Module Block Diagram

7.4 Method of Operation

Starting with the chip unpowered, the analog and digital power supplies turned on, the bandgap

voltage reference and the comparators will begin to function. The bandgap voltage reference will

apply a temperature and supply stable voltage reference to the positive inputs of the comparators.

Negative inputs of the comparators are connected to voltage points that move proportionately

with respect to the analog and digital power supplies.

Initially, the bandgap voltage reference point is greater than the power supply reference signals

and the output of the comparators is high. As each power supply goes above it’s trip point, the

voltage on the respective comparators negative input will become higher in value than the

bandgap voltage reference voltage on the positive input to the comparator and the output of the

comparator will go low. If either power supply drops below the trip point the respective POR

output will again go high.

For the analog power supply, the POR trip point is:

Absolute Minimum

Nominal

Absolute Maximum

2.8V

2.85V

2.9V

–

Bandgap

Voltage

Reference

Voltage

Level

Shifter

Voltage

Level

Shifter

Analog Power

GND

Digital Power

GND

POR_1.8

POR_3.3

DSP56857 Datasheet(PDF) 172 Page - Freescale Semiconductor, Inc