CS4218 datasheet(22/44 Pages) CIRRUS

22 / 44 page

SERIAL MODE 4, (SM4)

Se ria l Mo de 4

is en ab led by sett in g

SMODE3 = 1. Both Master and Slave sub-

modes are available and are selected by setting

the SMODE2 and SMODE1 pins as shown in

Table 6. In Master sub-mode, the phase relation-

ship between SCLK/SSYNC and CLKIN is

controlled to minimize digital noise coupling

into the analog section. Therefore, Master sub-

mo de may yield sl ightly better noise

performance than Slave sub-mode.

In Slave

sub-mode, SCLK and SSYNC must be synchro-

nous to CLKIN.

In serial mode 4, the CLKIN frequency must be

256 times the highest sample frequency needed.

SM4 differs from SM3 and SM5 in that SM4

splits the audio data from the control data, with

the control data on an independent serial port.

This reduces the audio serial bus bandwidth by

half, providing an easier interface to low-cost

DSPs. The audio serial port sub-frame is illus-

trated in Figure 19 for SM4.

Master Sub-Mode (SM4)

Master sub-mode configures SSYNC and SCLK

as outputs from the CS4218. During power

down, SSYNC and SCLK are driven high im-

pedance, and during reset they both are driven

low. There are two SM4 Master sub-modes. One

allows 32 bits per frame and the other allows 64

bits per frame. As shown in Table 6, the

SMODE1 and SMODE2 pins select the particu-

lar Master sub-mode (as well as the Slave

sub-mode). When SMODE1 is set to zero,

SMODE2 selects either Master sub-mode with

32-bit frames, or Slave sub-mode.

SMODE1,SMODE2 = 00 selects Master sub-

mode where a frame = sub-frame = 32 bits. This

sub-mode allows only one codec on the audio

serial bus, with the first 16 bits being the left

channel and the second 16 bits being the right

channel. The Appendix B section contains more

information on low-cost implementations of this

sub-mode.

SMODE1 = 1 selects Master sub-mode with a

frame width of 64 bits. This sub-mode allows up

to two codecs to occupy the same bus. SMODE2

is now used to select the particular time slot. If

SMODE2 = 0 the codec selects time slot 1,

which is the first 32 bits. If SMODE2 = 1 the

codec selects time slot 2, which is the second

32 bits.

SMODE1

SMODE2

SM4, Sub-Mode

Master, 32 BPF

Slave, 128/64/32 BPF

Master, 64 BPF, TS1

Master, 64 BPF, TS2

Table 6. SM4 Sub-Modes.

SSYNC

SCLK

SDIN

SDOUT

Sub-Frame

ADC - Left Word

ADC - Right Word

ADC - Left Word

ADC - Right Word

DAC - Left Word

DAC - Right Word

DAC - Left Word

DAC - Right Word

(slave)

(master)

Figure 19. SM4-Audio Serial Port, 32 BPF

CS4218

DS135F1

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