MCP3901A2-ISLASHML datasheet(31/60 Pages) MICROCHIP

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DS22192A-page 31

MCP3901

5.10.1

PHASE DELAY LIMITS

The Phase delay can only go from -OSR/2 to +OSR/2 - 1.

This sets the fine phase resolution. The phase register is

coded with 2's complement.

If larger delays between the two channels are needed,

they can be implemented externally to the chip with a

MCU. A FIFO in the MCU can save incoming data from

the leading channel for a number N of DRCLK clocks.

In this case, DRCLK would represent the coarse timing

resolution, and DMCLK the fine timing resolution. The

total delay will then be equal to:

Delay = N/DRCLK + PHASE/DMCLK

The Phase delay register can be programmed once

with the OSR=256 setting and will adjust to the OSR

automatically afterwards without the need to change

the value of the PHASE register.

• OSR=256: the delay can go from -128 to +127.

PHASE<7> is the sign bit. Phase<6> is the MSB

and PHASE<0> the LSB.

• OSR=128: the delay can go from -64 to +63.

PHASE<6> is the sign bit. Phase<5> is the MSB

and PHASE<0> the LSB.

• OSR=64: the delay can go from -32 to +31.

PHASE<5> is the sign bit. Phase<4> is the MSB

and PHASE<0> the LSB.

• OSR=32: the delay can go from -16 to +15.

PHASE<4> is the sign bit. Phase<3> is the MSB

and PHASE<0> the LSB.

5.11

Crystal Oscillator

The MCP3901 includes a Pierce type crystal oscillator

with very high stability and ensures very low tempco

and jitter for the clock generation. This oscillator can

handle up to 16.384 MHz crystal frequencies provided

that proper load capacitances and quartz quality factor

are used.

For keeping specified ADC accuracy, AMCLK should

be kept between 1 and 5 MHz with BOOST off or 1 and

8.192 MHz with BOOST on. Larger MCLK frequencies

can be used provided the prescaler clock settings allow

the AMCLK to respect these ranges.

For a proper start-up, the load capacitors of the crystal

should be connected between OSC1 and DGND and

between OSC2 and DGND. They should also respect

the following equation:

EQUATION 5-5:

where f is the crystal frequency in MHz, CLOAD, the load

capacitance in pF including parasitics from the PCB

and RM the motional resistance in ohms of the quartz

(it also defines the quality factor).

When CLKEXT=1, the crystal oscillator is bypassed by

a digital buffer to allow direct clock input for an external

clock (see Figure 1-5).

TABLE 5-8:

PHASE VALUES WITH

MCLK = 4 MHZ, OSR = 256

Phase Register

Value

Hex

Delay

(CH0 relative

to CH1)

011 111 11

0x7F

+ 127 µs

011 111 10

0x7E

+ 126 µs

000 000 01

0x01

+ 1 µs

000 000 00

0x00

0 µs

111 111 11

0xFF

- 1 µs

100 000 01

0x81

- 127 µs

100 000 00

0x80

-128 µs

1.6

LOAD

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