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Active Clock Frequency

The purpose of this advisory is to clarify the function of the serial I/O control registers in the DSP1620/27/28/29 devices. Specifically, it clarifies the function of the control register field that specifies the active clock frequency. The device data sheets state that the active clock frequency is a ratio of the input clock frequency on the CKI pin (DSP1627/28/29 devices) or the output clock frequency on the CKO pin (DSP1620 device). For all four devices, the actual active clock frequency is a ratio of the internal clock frequency, which can be programmed as either the input clock frequency on the CKI pin or the output of an internal clock synthesizer (PLL).