| |||

Home > Controllers and Peripherals > Synchronous Serial Port (SSP) > Functional description > Using an external reference clock |

In the slave mode of operation, the **SSPCLKIN** signal
from the external master is double synchronized and then delayed
to detect an edge. It takes three **SSPCLK**s
to detect an edge on **SSPCLKIN**. **SSPTXD** has less setup time to the falling
edge of **SSPCLKIN** on which the
master is sampling the line. The setup and hold times on **SSPRXD** with reference to **SSPCLKIN** must be more conservative
to ensure that it is at the right value when the actual sampling
occurs within the SSPMS. To ensure correct device operation, **SSPCLK** must be at least 12 times faster
than the maximum expected frequency of **SSPCLKIN**.

The frequency selected for **SSPCLK** must
accommodate the desired range of bit clock rates. The ratio of minimum **SSPCLK** frequency to **SSPCLKOUT** maximum frequency
in the case of the slave mode is 12 and for the master mode it is
two.

To generate a maximum bit rate of 1.8432Mbps in the Master
mode, the frequency of **SSPCLK** must
be at least 3.6864MHz. With an **SSPCLK** frequency
of 3.6864MHz, the SSPCPSR register has to be programmed with a value
of two and the SCR[7:0] field in the SSPCR0 register must be programmed
as zero.

To work with a maximum bit rate of 1.8432Mbps in the slave
mode, the frequency of **SSPCLK** must
be at least 22.12MHz. With an **SSPCLK** frequency
of 22.12MHz, the SSPCPSR register can be programmed with a value
of 12 and the SCR[7:0] field in the SSPCR0 register can be programmed
as zero. Similarly the ratio of **SSPCLK** maximum frequency
to **SSPCLKOUT** minimum frequency
is 254x256.

The minimum frequency of **SSPCLK** is
governed by the following equations, both of which have to be satisfied:

F** _{SSPCLK}**(min)
=> 2 x F

F** _{SSPCLK}**(min)
=> 12 x F

The maximum frequency of **SSPCLK** is
governed by the following equations, both of which have to be satisfied:

F** _{SSPCLK}**(max)
<= 254 x 256 x F

F** _{SSPCLK}**(max)
<= 254 x 256 x F