Data Transmission and Reception
11-32
11.3.4.8 Multiphase Frame Example: AC97
Figure 11–14 shows an example of the Audio Codec ’97 (AC97) standard,
which uses the dual-phase frame feature. The first phase consists of a single
16-bit element. The second phase consists of 12 20-bit elements. The phases
are configured as follows:
-
(R/X)PHASE = 1b: specifying a dual-phase frame
-
(R/X)FRLEN1 = 0b: specifying one element per frame in phase 1
-
(R/X)WDLEN1 = 010b: specifying 16 bits per element in phase 1
-
(R/X)FRLEN2 = 0001011b: specifying 12 elements per frame in phase 2
-
(R/X)WDLEN2 = 011b: specifying 20 bits per element in phase 2
-
CLK(R/X)P = 0: specifying that the receive data sampled on the falling edge
of CLKR and the transmit data are clocked on the rising edge of CLKX
-
FS(R/X)P = 0: indicating that active frame sync signals are used
-
(R/X)DATDLY = 01b: indicating a data delay of one bit clock
Figure 11–14.
AC97 Dual-Phase Frame Format
{
D(R/X)
FS(R/X)
P2E12
P2E11
P2E10
P2E9
P2E8
P2E7
P2E6
P2E5
P2E4
P2E3
P2E2
P2E1
P1E1
20 bits
16 bits
1-bit data delay
Á
Á
† PxEy denotes phase x and element y.
Figure 11–14 shows the AC97 timing near frame synchronization. First the
frame sync pulse itself overlaps the first element. In McBSP operation, the inac-
tive-to-active transition of the frame synchronization signal actually indicates
frame synchronization. For this reason, frame synchronization can be high for
an arbitrary number of bit clocks. Only after the frame synchronization is recog-
nized as inactive and then active again is the next frame synchronization recog-
nized.
In Figure 11–15, there is 1-bit data delay. Regardless of the data delay, trans-
mission can occur without gaps. The last bit of the previous (last) element in
phase 2 is immediately followed by the first data bit of the first element in phase
1 of the next data frame.