Integer Multiply and Divide Programming Example

Zynq UltraScale+ Device Technical Reference Manual (UG1085)

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2.4 English

This example assumes the input PS_REF_CLK frequency is 50 MHz, the [FBDIV] value is 40, and the output divider is turned on. The output clock is calculated to be 50 MHz x 40/2 = 1000 MHz. For a new frequency of 1600 MHz, the [FBDIV] value is switched to 32 and the output divider is turned off. This example uses the APLL.

Note:   Before reprogramming the PLL clock output frequency, check that the downstream clocks are in a safe state before releasing. For instance, if the APU DIVISOR is set to 2.

1.Program the new FBDIV, CLKOUT value (do not modify other values in the APLL_CTRL register).

Set APLL_CTRL = 0000_2000h: [DIV2] = 0, [FBDIV] = 20h.

2.Program the helper data for APLL_CFG using the helper data in Table: PLL Integer Feedback Divider Helper Data Values.

3.Program the bypass.

Set APLL_CTRL = 0000_2008h: [BYPASS] = 1.

4.Assert reset. This is when the new data is actually captured into the PLL.

Set APLL_CTRL = 0000_2009h: [RESET] = 1.

5.Deassert reset.

Set APLL_CTRL = 0000_2008h: [RESET] = 0.

6.Check for LOCK. Wait until: PLL_STATUS [APLL_LOCK] = 1

7.Deassert bypass.

Set APLL_CTRL = 0000_2000h: [BYPASS] = 20h.

The PLL output clock is set to 1600 MHz.