Dual Modulus Prescaler

Integer clock divider with two divider ratios

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Libraries:
Mixed-Signal Blockset / PLL / Building Blocks

Description

The Dual Modulus Prescaler subsystem block consists of a program counter, a swallow counter and a prescaler.

When the block first receives an input signal, the pulse swallow function is activated. The prescaler divides the input signal frequency by (N+1), where N is defined by the Prescaler divider value (N) parameter. Both the program and swallow counters start counting. The swallow counter resets after counting to S pulses, or (N+1)S cycles, where S is defined by the Swallow counter value (S) parameter. Then, the pulse swallow function is deactivated, and the prescaler divides the input frequency by N.

Since the program counter has already sensed S pulses, it requires (P-S) more pulses, or (P-S)N cycles to reach overflow, where P is defined by the Program counter value (P) parameter. The cycle repeats after both counters are reset.

The effective divider value of the dual modulus prescaler is the ratio of the input frequency to the output frequency:

$$\frac{f_{\text{in}}}{f_{\text{out}}}=(N+1)S+N(P-S)=NP+S$$

Note

To prevent the program counter and prescaler from resetting prematurely before the swallow counter finishes counting, the condition $$P\ge S$$ must be met.

The dual modulus prescaler is also known as pulse swallow divider.

Examples

Frequency Division Using Dual Modulus Prescaler

Open the model Dual_Modulus_Prescaler_Ex. The model consists of a Pulse Generator and a Dual Modulus Prescaler block.

• Open Script

Ports

Input

clk in — Input clock frequency

scalar

Input clock frequency, specified as a scalar. In a phase-locked loop (PLL) system, the clk in port is connected to the output port of a VCO block.

Data Types: double

Output

clk out — Output clock frequency

scalar

Output clock frequency, specified as a scalar. In a PLL system, the clk out port is connected to the feedback input port of a PFD block. The output at the clk out port is a square pulse train of 1 V amplitude.

Data Types: double

Parameters

Program counter value, P — Maximum value of program counter

12 (default) | scalar integer

Maximum value of the program counter, specified as a scalar integer. The counter resets after P cycles. This parameter is tunable.

Programmatic Use

• Use get_param(gcb,'P') to view the current Program counter value value.

• Use set_param(gcb,'P',value) to set Program counter value to a specific value.

Prescaler divider value, N — Prescaler divider value

4 (default) | scalar integer

Prescaler divider value, specified as a scalar integer. A N/(N+1) dual modulus prescaler divides the input frequency by either N or N+1, depending on the logical state of modulus control line. This parameter is tunable.

Programmatic Use

• Use get_param(gcb,'N') to view the current Prescaler divider value.

• Use set_param(gcb,'N',value) to set Prescaler divider value to a specific value.

Swallow counter value, S — Maximum value of swallow counter

2 (default) | scalar integer

Maximum value of the swallow counter, specified as a scalar integer. When the swallow counter resets after S cycles, the pulse swallow function is deactivated.

Programmatic Use

• Use get_param(gcb,'S') to view the current Swallow counter value value.

• Use set_param(gcb,'S',value) to set Swallow counter value to a specific value.

Enable increased buffer size — Enable increased buffer size

off (default) | on

Select to enable increased buffer size during simulation. This increases the buffer size of the Logic Decision inside the Dual Modulus Prescaler block. By default, this option is deselected.

Buffer size — Number of samples of the input buffering available during simulation

1 (default) | positive integer scalar

Number of samples of the input buffering available during simulation, specified as a positive integer scalar. This sets the buffer size of the Logic Decision inside the Dual Modulus Prescaler block.

Selecting different simulation solver or sampling strategies can change the number of input samples needed to produce an accurate output sample. Set the Buffer size to a large enough value so that the input buffer contains all the input samples required.

Dependencies

This parameter is only available when Enable increased buffer size option is selected in the Block Parameters dialog box.

Programmatic Use

• Use get_param(gcb,'NBuffer') to view the current value of Buffer size.

• Use set_param(gcb,'NBuffer',value) to set Buffer size to a specific value.

More About

Inside the Mask

The Dual Modulus Prescaler subsystem block consists of three different subsystems that implement the three main parts of the dual modulus prescaler. The Prescaler divides the input frequency by either N or N+1, depending on the logical state of modulus control line. The Program Counter subsystem always divides the prescaler output frequency by P.

The Swallow Counter divides the prescaler output by S. S depends on the digital input and can vary from 1 to maximum number of channels. S also determines the logic state of the modulus control line.

References

[1] Razavi, Behzad. RF Microelectronics. Upper Saddle River, NJ: Prentice Hall PTR, 1998.

Version History

Introduced in R2019a