CS4218 Datasheet, PDF (22/44 Pages) Cirrus Logic – 16-Bit Stereo Audio Codec

CS4218
SERIAL MODE 4, (SM4)
Serial Mode 4 is enabled by setting
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-
mode may yield slightly 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.
SMODE1
0
0
1
1
SMODE2
0
1
0
1
SM4, Sub-Mode
Master, 32 BPF
Slave, 128/64/32 BPF
Master, 64 BPF, TS1
Master, 64 BPF, TS2
Table 6. SM4 Sub-Modes.
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.
(master)
SSYNC
(slave)
SCLK
Sub-Frame
SDOUT
ADC - Right Word
SDIN
DAC - Right Word
22
ADC - Left Word
ADC - Right Word
DAC - Left Word
DAC - Right Word
Figure 19. SM4-Audio Serial Port, 32 BPF
ADC - Left Word
DAC - Left Word
DS135F1