WM8978 datasheet(64/78 Pages) WOLFSON

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WM8978

Product Preview

PP Rev 1.7 January 2005

Figure 31 3-Wire Serial Control Interface

2-WIRE SERIAL CONTROL MODE

The WM8978 supports software control via a 2-wire serial bus. Many devices can be controlled by

the same bus, and each device has a unique 7-bit device address (this is not the same as the 7-bit

address of each register in the WM8978).

The WM8978 operates as a slave 2-wire device only. The controller indicates the start of data

transfer with a high to low transition on SDIN while SCLK remains high. This indicates that a device

address and data will follow. All devices on the 2-wire bus respond to the start condition and shift in

the next eight bits on SDIN (7-bit address + Read/Write bit, MSB first). If the device address

received matches the address of the WM8978, then the WM8978 responds by pulling SDIN low on

the next clock pulse (ACK). If the address is not recognised or the R/W bit is ‘1’ when operating in

write only mode, the WM8978 returns to the idle condition and wait for a new start condition and

valid address.

During a write, once the WM8978 has acknowledged a correct address, the controller sends the first

byte of control data (B15 to B8, i.e. the WM8978 register address plus the first bit of register data).

The WM8978 then acknowledges the first data byte by pulling SDIN low for one clock pulse. The

controller then sends the second byte of control data (B7 to B0, i.e. the remaining 8 bits of register

data), and the WM8978 acknowledges again by pulling SDIN low.

Transfers are complete when there is a low to high transition on SDIN while SCLK is high. After a

complete sequence the WM8978 returns to the idle state and waits for another start condition. If a

start or stop condition is detected out of sequence at any point during data transfer (i.e. SDIN

changes while SCLK is high), the device jumps to the idle condition.

SDIN

SCLK

1st register data bit

DEVICE ADDRESS

(7 BITS)

RD / WR

BIT

ACK

(LOW)

CONTROL BYTE 1

(BITS 15 TO 8)

CONTROL BYTE 1

(BITS 7 TO 0)

remaining 8 bits of

STOP

START

ACK

(LOW)

ACK

(LOW)

Figure 32 2-Wire Serial Control Interface

In 2-wire mode the WM8978 has a fixed device address, 0011010.

RESETTING THE CHIP

The WM8978 can be reset by performing a write of any value to the software reset register (address

0 hex). This will cause all register values to be reset to their default values. In addition to this there

is a Power-On Reset (POR) circuit which ensures that the registers are set to default when the

device is powered up.

POWER SUPPLIES

The WM8978 can use up to four separate power supplies:

AVDD and AGND: Analogue supply, powers all analogue functions except the speaker output and

mono output drivers. AVDD can range from 2.5V to 3.6V and has the most significant impact on

overall power consumption (except for power consumed in the headphone). A large AVDD slightly

improves audio quality.

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