WM8255 datasheet(35/49 Pages) CIRRUS

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WM8255

Production Data

PD, Rev 4.7, August 2013

PROGRAMMABLE VSMP DETECT CIRCUIT

The VSMP input is used to determine the sampling point and frequency of the WM8255. Under

normal operation a pulse of 1 MCLK period should be applied to VSMP at the desired sampling

frequency (as shown in the Operating Mode Timing Diagrams) and the input sample will be taken on

the first rising MCLK edge after VSMP has gone low. However, in certain applications such a signal

may not be readily available. The programmable VSMP detect circuit in the WM8255 allows the

sampling point to be derived from any signal of the correct frequency, such as a CCD shift register

clock, when applied to the VSMP pin.

When enabled, by setting the VSMPDET control bit, the circuit detects either a rising or falling edge

(determined by POSNNEG control bit) on the VSMP input pin and generates an internal VSMP pulse.

This pulse can optionally be delayed by a number of MCLK periods, specified by the VDEL[2:0] bits.

Figure 27 shows the internal VSMP pulses that can be generated by this circuit for a typical clock

input signal. The internal VSMP pulse is then applied to the timing control block in place of the normal

VSMP pulse provided from the input pin. The sampling point then occurs on the first rising MCLK

edge after this internal VSMP pulse, as shown in the Operating Mode Timing Diagrams.

MCLK

VSMP

(VDEL = 000) INTVSMP

POSNNEG = 1

(VDEL = 001) INTVSMP

(VDEL = 010) INTVSMP

(VDEL = 011) INTVSMP

(VDEL = 100) INTVSMP

(VDEL = 101) INTVSMP

(VDEL = 110) INTVSMP

(VDEL = 111) INTVSMP

POSNNEG = 0

(VDEL = 000) INTVSMP

(VDEL = 001) INTVSMP

(VDEL = 010) INTVSMP

(VDEL = 011) INTVSMP

(VDEL = 100) INTVSMP

(VDEL = 101) INTVSMP

(VDEL = 110) INTVSMP

(VDEL = 111) INTVSMP

INPUT

PINS

Figure 27 Internal VSMP Pulses Generated by Programmable VSMP Detect Circuit

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