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AN258

EP93xx Power-up and Reset Lockup Workaround

1. Introduction

Under certain circumstances, the EP93xx may enter into a “locked” condition on power up, when a soft-

ware reset is issued, or when an internal watchdog reset is issued. This application note provides a low-

cost workaround which will provide reliable power up and reset operation by adding a few readily-avail-

able components. A fix of this issue is planned in a future revision of the silicon.

When new silicon becomes available, applications that employ this workaround should not have to be re-

designed. Customers could use the same board design and simply not populate the additional compo-

nents except a single zero-ohm resistor.

2. Workaround

The workaround circuits take advantage of the one thing that locked conditions have in common: the red

LED stays illuminated. The basic idea of the circuits is to detect the red LED staying on for a period of

time and then to issue a manual reset to the voltage monitor IC that drives POR after an appropriate

amount of time has elapsed.

These hardware workarounds will detect this locked condition and restart the EP93xx device. It requires

a few passive devices and either a Schmitt-trigger inverter or a good-quality 2-input NAND with Schmitt

trigger. A simplified schematic of each of the proposed circuits is shown in Figure 1 and Figure 2.

Figure 1. Proposed Reset Circuit 1

RDLED

JUMPER

1 2

LED1

RED_LED

U2A

74HC14/SO

1 2

2.2uF

R389

10K

0.1uF

100K

560

/POR

(DEFAULT=INSTALL)

MAX708CSA

4 5

/MR

Vcc

GND

PFI /PFO

/RST

RST

0.1uF

Oct ‘04

AN258REV2

1 2 3 4 5 6

Resumo do Conteúdo

Página 1 - 2. Workaround

Página 2 - 2 AN258REV2

AN2582 AN258REV2Figure 2. Proposed Reset Circuit 2U6MAX708C SA1234 5678/MRVccGNDPFI /PFONC/RSTRST5VR4Zero Ohm JumperR5560R1100KU7C74VHC1329108U7B74VH

Página 3 - IMPORTANT

AN258AN258REV2 33. Reset Circuit OverviewThe circuits shown in Figure 1 and Figure 2 are proposed circuits to keep the EP93xx device from beingheld in

Página 4 - 4 AN258REV2

AN2584 AN258REV2Figure 3. Oscilloscope View of the Red LED RC Circuit, R2 = 120 kΩThe second part of the circuit causes a simulated manual reset butt

Página 5 - AN258REV2 5

AN258AN258REV2 5Figure 4. Oscilloscope View of the RC Circuit at the MR PinThis figure shows channel 1, the output of the Schmitt-trigger inverter in

Página 6

AN2586 AN258REV2Revision Date Changes1 27 Sep 2004 Initial Release2 30 Sep 2004 Added alternate circuit (Circuit 2).Contacting Cirrus Logic SupportFor

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