30F4013 datasheet(133/228 Pages) MICROCHIP

133 / 228 page

 2010 Microchip Technology Inc.

DS70138G-page 133

dsPIC30F3014/4013

19.4

Programming the Start of

Conversion Trigger

The conversion trigger terminates acquisition and

starts the requested conversions.

The SSRC<2:0> bits select the source of the conver-

sion trigger. The SSRC bits provide for up to 4 alternate

sources of conversion trigger.

When SSRC<2:0> = 000, the conversion trigger is

under software control. Clearing the SAMP bit causes

the conversion trigger.

When SSRC<2:0> = 111 (Auto-Convert mode), the

conversion trigger is under A/D clock control. The

SAMC bits select the number of A/D clocks between

the start of acquisition and the start of conversion. This

provides the fastest conversion rates on multiple

channels. The SAMC bits must always be at least one

clock cycle.

Other trigger sources can come from timer modules or

external interrupts.

19.5

Aborting a Conversion

Clearing the ADON bit during a conversion aborts the

current conversion and stops the sampling sequencing

until the next sampling trigger. The ADCBUF is not

updated with the partially completed A/D conversion

sample. That is, the ADCBUF will continue to contain

the value of the last completed conversion (or the last

value written to the ADCBUF register).

If clearing of the ADON bit coincides with an auto-start,

the clearing has a higher priority and a new conversion

does not start.

19.6

Selecting the ADC Conversion

Clock

The ADC conversion requires 14 TAD. The source of

the ADC conversion clock is software selected, using a

6-bit counter. There are 64 possible options for TAD.

EQUATION 19-1:

ADC CONVERSION

CLOCK

The internal RC oscillator is selected by setting the

ADRC bit.

For correct ADC conversions, the ADC conversion

clock (TAD) must be selected to ensure a minimum TAD

time of 334 nsec (for VDD = 5V). Refer to Section 23.0

“Electrical Characteristics” for minimum TAD under

other operating conditions.

Example 19-1 shows a sample calculation for the

ADCS<5:0> bits, assuming a device operating speed

of 30 MIPS.

EXAMPLE 19-1:

ADC CONVERSION

CLOCK AND SAMPLING

RATE CALCULATION

TAD = TCY * (0.5*(ADCS<5:0> + 1))

Minimum TAD = 334 nsec

ADCS<5:0> = 2

– 1

TAD

TCY

TCY = 33.33 nsec (30 MIPS)

= 2 •

– 1

334 nsec

33.33 nsec

= 19

Therefore,

Set ADCS<5:0> = 19

Actual TAD =

(ADCS<5:0> + 1)

TCY

(19 + 1)

33.33 nsec

= 334 nsec

If SSRC<2:0> = 111 and SAMC<4:0> = 00001

Since,

Sampling Time = Acquisition Time + Conversion Time

= 1 TAD + 14 TAD

= 15 x 334 nsec

Therefore,

Sampling Rate =

= ~200 kHz

(15 x 334 nsec)

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