ID 683777
Date 9/26/2022
Public

## 4.2.13.1.1. Post-Scale Counters (C0 to C4)

You can configure the multiply or divide values and duty cycle of the post-scale counters in real time. Each counter has an 8-bit high time setting and an 8-bit low time setting. The duty cycle is the ratio of output high or low time to the total cycle time, which is the sum of the two.

The post-scale counters have two control bits:

• rbypass—For bypassing the counter
• rselodd—For selecting the output clock duty cycle

When the rbypass bit is set to 1, it bypasses the counter, resulting in a division by one. When this bit is set to 0, the PLL computes the effective division of the VCO output frequency based on the high and low time counters. The PLL implements this duty cycle by transitioning the output clock from high-to-low on the rising edge of the VCO output clock.

For example, if the post-scale divide factor is 10, the high and low count values are set to 5 and 5 respectively, to achieve a 50–50% duty cycle. However, a 4 and 6 setting for the high and low count values, respectively, would produce an output clock with 40–60% duty cycle.

The rselodd bit indicates an odd divide factor for the VCO output frequency with a 50% duty cycle. The PLL implements this duty cycle by transitioning the output clock from high-to-low on a falling edge of the VCO output clock.

For example, if the post-scale divide factor is 3, the high and low time count values are 2 and 1 respectively, to achieve this division. This implies a 67%–33% duty cycle. If you need a 50%–50% duty cycle, you must set the rselodd control bit to 1 to achieve this duty cycle despite an odd division factor. When you set rselodd = 1, subtract 0.5 cycles from the high time and add 0.5 cycles to the low time.

The calculation for the example is shown as follows:

• High time count = 2 cycles
• Low time count = 1 cycle
• rselodd = 1 effectively equals:
• High time count = 1.5 cycles
• Low time count = 1.5 cycles
• Duty cycle = (1.5/3)% high time count and (1.5/3)% low time count