National Instruments NI USB-6509 Change Detection Example, Watchdog Timer

Change Detection Example

Table4 shows a change detection example for six bits of one port.

This example assumes the following line connections:

• Bits 7, 6, 5, and 4 are connected to data lines from a four-bit TTL

output device. The NIUSB-6509 det ects any change in the input data

so you can read the new data value.

• Bit 1 is connected to a limit sensor. The NIUSB-6509 detects rising

edges on the sensor, which correspond to over-limit conditions.

• Bit 0 is connected to a switch. The software can react to any switch

closure, which is represented by a falling edge. If the switch closure is

noisy, enable digital filtering for this line.

In this example, the NI USB-6509 reports rising edges only on bit 1, falling

edges only on bit 0, and rising and falling edges on bits 7, 6, 5, and 4.

The NI USB-6509 reports no changes for bits 3 and 2. After receiving

notification of a change, you can read the port to determine the current

values of all eight lines. You cannot read the state of any lines that are

configured for change detection until the change detection interrupt occurs.

Watchdog Timer

The watchdog timer is a software configurable feature used to set critical

outputs to safe states in the event of a software failure, a system crash, or

any other loss of communication between the application and the

NIU SB-6509.

When the watchdog timer is enabled, if the NI USB-6509 does not receive

a watchdog reset software command within the time specified for the

watchdog timer, the outputs go to a user-defined safe state and remain in

that state until the watchdog timer is disarmed by the application and new

values are written, the NIUSB-6509 is reset, or the computer is restarted.

Table 4. Change Detection Example

Bit

76543210

Changes to detect — —

Enable rising-edge

detection

yes yes yes yes no no yes no

Enable falling-edge

detection

yes yes yes yes no no no yes