Cirrus Logic AN258 User Manual
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 software reset is issued, or when an internal watchdog reset is issued. This application note provides a lowcost workaround which will provide reliable power up and reset operation by adding a few readily-available 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 redesigned. Customers could use the same board design and simply not populate the additional components except a single zero-ohm resistor.
2. Workaround
The workaround circuits take advantage of the one thing that locked conditio ns have in common: the re d
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.
RDLED
R2
R
R5
560
C2
2.2uF
LED1
RED_LED
5V
R1
100K
C1
(DEFAULT=INSTALL)
0.1uF
U2A
1 2
74HC14/SO
5V
C3
0.1uF
Figure 1. Proposed Reset Circuit 1
D1
J1
1 2
JUMPER
U6
1
/MR
2
Vcc
3
GND
4 5
PFI /PFO
MAX708CSA
RST
/RST
NC
R389
10K
8
7
6
/POR
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Copyright © Cirrus Logic, Inc. 2004
(All Rights Reserved)
Oct ‘04
AN258REV2
1
RDLED
SW2
PUSHBUTTON
R2
R
C2
2.2uF
U7A
1
2
74HVC132
AN258
R4
U7B
5
4
74VHC132
3
Zero Ohm Jumper
(DO N OT POPU L ATE)
(SEE APP NOTE)
9
6
10
5V
C3
0.1uF
U7C
74VHC132
100K
5V
R1
D1
C1
8
0.1uF
J1
1 2
JU MPER
(DEFAULT=INSTALL)
5V
U6
1
2
3
4 5
MAX708C SA
/MR
RST
Vcc
/RST
GND
PFI /PFO
R389
10K
8
7
6
NC
/POR
5V
R3
10K
R5
560
LED1
RED_LED
Figure 2. Proposed Reset Circuit 2
2 AN258REV2
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