SPRUI30H November 2015 – May 2024 DRA745 , DRA746 , DRA750 , DRA756
The serial bitstream transmitted or received by a McASP serializer is a long sequence of 1s and 0s on an audio transmit/receive pins AXRn. However, the sequence has a hierarchical organization that can be described in terms of frames of data, slots, words, and bits.
A basic synchronous serial interface consists of three important components: clock, frame sync, and data. Figure 24-110 shows two of the three basic components: the clock signal (ACLKX/ACLKR) and the data signals AXRn. Figure 24-110 does not specify whether the clock is for transmit (ACLKX) or receive (ACLKR) because the definitions of terms apply to both receive and transmit interfaces. In operation, each transmitter and receiver uses the signals ACLKX and ACLKR as serial clock, respectively. Optionally, a receiver can use ACLKX as the serial clock when a transmitter and receiver (not from the same serializer) of the McASP are configured to operate synchronously.
A bit is the smallest entity in the serial data stream. The beginning and end of each bit is marked by an edge of the serial clock. The duration of a bit is a serial clock period. A '1' is represented by a logic high on AXRn pins for the entire duration of the bit. A 0 is represented by a logic low on an AXRn pin for the entire duration of the bit.
A word is a group of bits that make up the data being transferred between the McASP and the external device. Figure 24-110 shows an 8-bit word.
A slot consists of the bits that make up the word and can consist of additional bits used to pad the word to a convenient number of bits for the interface between the McASP and the external device. In Figure 24-110, the audio data consists of only 8 bits of useful data (8-bit word), but it is padded with four 0s (12-bit slot) to satisfy the desired protocol in interfacing to an external device. Within a slot, the bits can be shifted out of the McASP on an AXRn pin with either MSB or LSB first.
When the word size is smaller than the slot size, the word can be aligned to the left of the slot (beginning) or to the right of the slot (end). The additional bits in the slot not belonging to the word can be padded with 0, 1, or with one of the bits (typically, the MSB or LSB) from the data word, i.e. left-aligned words within a slot are terminated with padding bits and right-aligned words within a slot are preceded by padding bits to fit in the slot size. Figure 24-111 shows these options.
The third basic element of a synchronous serial interface is the frame synchronization signal, also referred to as frame sync in this chapter. A frame contains one or multiple slots, as determined by the desired protocol. Figure 24-112 shows an example frame of data and the frame definitions. In operation, the transmitter uses AFSX, and the receiver - AFSR signal. Figure 24-112 does NOT specify whether the frame sync (FS) is for transmit (AFSX) or receive (AFSR) because the definitions of terms apply to both receive and transmit interfaces. In operation, each transmitter/receiver uses AFSX/AFSR as a frame synchronization signal, respectively. Optionally, the receiver can use AFSX as the frame sync when the transmitter and receiver of the McASP are configured to operate synchronously. This example shows two slots in a frame (I2S format) and a frame-sync (FS) duration of a slot length.
This section shows only the generic definition of the frame sync. For more information about the frame-sync formats required for the transfer modes and protocols (TDM-mode and DIT-mode supported formats), see Section 24.6.2.2.3, TDM Format and Section 24.6.2.2.5, S/PDIF-Coding Format.
All of the McASP serializers share the same, device pad accessible, clock and frame signals, as follows:
The following terms are used throughout this chapter: