# Configuring the secondary clock master

You can optionally set up a secondary clock master to drive a CT bus clock if the primary clock master stops driving its CT bus clock. Use the following keywords to configure the secondary clock master:

Keyword | Description
--- | ---
Clocking.HBus.ClockMode | Specifies the CT bus clock that the board drives. For the secondary clock master, specify the clock not driven by the primary clock master. For example, if the primary master drives B_CLOCK, specify MASTER_A for this keyword for the secondary master.
Clocking.HBus.ClockSource | Specifies the primary timing reference for the board. For the secondary clock master, set to the CT bus clock driven by the primary master: A_CLOCK or B_CLOCK. This makes the secondary master a slave to the primary master.
Clocking.HBus.AutoFallBack | Enables or disables clock fallback on the board. For the secondary clock master, set to YES.
Clocking.HBus.FallBackClockSource | Specifies the fallback timing reference for the board to use if the primary timing reference fails. Once the secondary master is driving the CT bus clock, it continues to drive the clock until software intervention by an application. For the secondary clock master, set to any timing reference not used by the primary clock master: NETREF to use NETREF (also called NETREF1) NETREF2 to use NETREF2 (H.110 only) NETWORK to derive the timing from the clock pulse on a digital trunk connected to the board OSC to use the board's on-board oscillator. Use only when no other source is available.
Clocking.HBus.FallBackNetwork | If Clocking.HBus.FallBackClockSource is set to NETWORK, specifies the board trunk from which to derive the fallback timing reference.
Clocking.HBus.NetRefSpeed | If using NETREF(1) or NETREF2 as a timing reference, set to 8K.