Cirrus Logic AN258 - Manual (Page 3)

AN258
AN258REV2 3
3. Reset Circuit Overview
The circuits shown in Figure 1 and Figure 2 are proposed circuits to keep the EP93xx device from being
held in the locked state on power up or when a software reset or watchdog reset is issued. There may be
other circuits that can achieve the same function. Readers are encouraged to read the Reset Circuit Op-
eration section.
Each of these circuits closely resemble the reset circuit already on the Cirrus Logic development boards.
The components inside the L-shaped box augment the existing reset circuit to restart the board if a locked
condition is detected. The devices outside the dashed box are components that already exist on the Cir-
rus Logic development boards. The use of the Max708 is not mandatory as other voltage monitor ICs may
perform just as well.
The main difference between the two proposed circuits is that Circuit 2 incorporates the use of a pushbut-
ton but Circuit 1 does not. A push button is not always needed and in those cases, Circuit 1 may be pre-
ferred.
4. Reset Circuit Operation
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 issue a manual reset to the voltage monitor IC that drives POR after an appropriate amount of
time period has elapsed.
IMPORTANT
These circuits put a restriction on the use of the red LED. If the customer application uses this
LED for indication then the proposed circuit will issue a reset once the timeout period is reached,
otherwise this workaround has no impact on the application. The red LED may still be used in cus-
tomer applications, but it must not be left on continuously. It may be blinked quickly with suffi-
ciently long periods of off time to discharge the circuit.
There are two basic parts to each of the circuits:
- An RC circuit driven by the red LED pin from the EP93xx device and a Schmitt-trigger inverter or the NAND
package mentioned earlier.
- An RC circuit on the manual RESET pin on the voltage monitor IC that drives POR. An over-voltage diode
may be necessary if the voltage monitor IC does not have internal protection.
The first part of the circuit is quite simple. A typical value for R2 would be 120 k
Ω to 180 kΩ. This value
will produce a timeout value of approximately 1 to 1.5 seconds. Meaning, if the red LED stays lit for 1 to
1.5 seconds then the circuit will issue a manual reset. This causes the voltage monitor IC to issue a POR
and reboot the EP93xx device.
The value for R2 may be adjusted per the customer's application. Refer to Figure 3 for an oscilloscope
view of the red LED RC circuit charging up and the output of the Schmitt-trigger inverter in Circuit 1. Chan-
nel 1 is the RC and channel 2 is the output of the Schmitt-trigger inverter. Figure 3 shows that the Schmitt-
trigger inverter goes low once the RC reaches approximately 3V. The RC circuit will continue to charge
after the POR reset is issued and the EP93xx boots correctly the next time. The red LED is turned off by
the Boot ROM code if doing an internal boot. If an external boot is performed, it is up to the customer's
software to turn off the red LED.
Depending on the voltage monitor IC used, diode D1 may or may not be required. The problem is that the
voltage on C1 nearest the manual reset pin can reach up to 2*Vcc. If the voltage monitor IC does not have
an internal protection device, then an external diode is needed to protect the manual reset pin input.