May 1999, Issue 106

Dallas 1-Wire Devices, (Part 2):
All on One

SLAVE AS MASTER

The only time a slave 1-wire device can try to take over the bus is when it needs to indicate an interrupt event. Some 1-wire devices are capable of indicating an alarm condition. The alarm can be a bit in a device’s register that indicates a certain condition. The master might poll devices looking for these conditions, or if the device is interrupt enabled, it can issue two types of master-like bus operations.

A type-1 interrupt enables the slave to immediately signal an interrupt on the 1-wire bus as long as the master has left the bus in a reset state (i.e., issued a bus reset with no following command). A type-2 interrupt is withheld until the next bus reset.

Interrupts can hold the bus low for almost 500 ms. It’s easy for the master to pick up the interrupt without other 1-wire devices being affected because the master’s reset pulse has no maximum time limit for holding the bus low.

Using interrupts doesn’t in any way signal which device initiated the interrupt. It remains the master’s responsibility to poll devices to determine which device is signaling.