PIC16F1507-ESLASHSS. datasheet(170/278 Pages) MICROCHIP

170 / 278 page

PIC16(L)F1507

DS40001586D-page 170

 2011-2015 Microchip Technology Inc.

21.0

NUMERICALLY CONTROLLED

OSCILLATOR (NCO) MODULE

The Numerically Controlled Oscillator (NCOx) module

is a timer that uses the overflow from the addition of an

increment value to divide the input frequency. The

advantage of the addition method over simple counter

driven timer is that the resolution of division does not

vary with the divider value. The NCOx is most useful for

applications that require frequency accuracy and fine

resolution at a fixed duty cycle.

Features of the NCOx include:

• 16-bit increment function

• Fixed Duty Cycle (FDC) mode

• Pulse Frequency (PF) mode

• Output pulse width control

• Multiple clock input sources

• Output polarity control

• Interrupt capability

Figure 21-1 is a simplified block diagram of the NCOx

module.

21.1

NCOx Operation

The NCOx operates by repeatedly adding a fixed value

to an accumulator. Additions occur at the input clock rate.

The accumulator will overflow with a carry periodically,

which is the raw NCOx output (NCO_overflow). This

effectively reduces the input clock by the ratio of the

addition value to the maximum accumulator value. See

Equation 21-1.

The NCOx output can be further modified by stretching

the pulse or toggling a flip-flop. The modified NCOx

output is then distributed internally to other peripherals

and optionally output to a pin. The accumulator

overflow also generates an interrupt (NCO_interrupt).

The NCOx period changes in discrete steps to create

an average frequency. This output depends on the

ability of the receiving circuit (i.e., CWG or external

resonant converter circuitry) to average the NCOx

output to reduce uncertainty.

21.1.1

NCOx CLOCK SOURCES

Clock sources available to the NCOx include:

•HFINTOSC

•FOSC

• LC1_out

• CLKIN pin

The NCOx clock source is selected by configuring the

NxCKS<2:0> bits in the NCOxCLK register.

21.1.2

ACCUMULATOR

The accumulator is a 20-bit register. Read and write

access to the accumulator is available through three

registers:

• NCOxACCL

• NCOxACCH

• NCOxACCU

21.1.3

ADDER

The NCOx adder is a full adder, which operates

independently from the system clock. The addition of the

previous result and the increment value replaces the

accumulator value on the rising edge of each input clock.

21.1.4

INCREMENT REGISTERS

The increment value is stored in two 8-bit registers

making up a 16-bit increment. In order of LSB to MSB

they are:

• NCOxINCL

• NCOxINCH

When the NCO module is enabled, the NCOxINCH

should be written first, then the NCOxINCL register.

Writing to the NCOxINCL register initiates the incre-

ment buffer registers to be loaded simultaneously on

the second rising edge of the NCOx_clk signal.

The registers are readable and writable. The increment

registers are double-buffered to allow value changes to

be made without first disabling the NCOx module.

When the NCO module is disabled, the increment

buffers are loaded immediately after a write to the

increment registers.

EQUATION 21-1:

Note:

The increment buffer registers are not

user-accessible.

FOVERFLOW

NCO Clock Frequency

Increment Value



----------------------------------------------------------------------------------------------------------------

n = Accumulator width in bits

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