b7
b6
b5 b4
b3
b2 b1
b0
P
P
P
P
bit
ACLK
AXR[n]
word
slot
1089
SPRUH82C – April 2013 – Revised September 2016
Copyright © 2013–2016, Texas Instruments Incorporated
Multichannel Audio Serial Port (McASP)
24.0.19 Definition of Terms
The serial bit stream transmitted or received by the McASP is a long sequence of 1s and 0s, either output
or input on one of the audio transmit/receive pins (AXR[n]). 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.
shows two of the three basic components—the clock (ACLK) and the data (AXR[n]).
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, the transmitter uses ACLKX
as the serial clock, and the receiver uses ACLKR as the serial clock. Optionally, the receiver can use
ACLKX as the serial clock when the transmitter and receiver of the McASP are configured to operate
synchronously.
Bit
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 the AXR[n] pin for the
entire duration of the bit. A 0 is represented by a logic low on the AXR[n] pin for the entire duration of the bit.
Word
A word is a group of bits that make up the data being transferred between the CPU and the external device.
shows an 8-bit word.
Slot
A slot consists of the bits that make up the word, and may consist of additional bits used to pad the word to a
convenient number of bits for the interface between the CPU and the external device. In
, the audio
data consists of only 8 bits of useful data (8-bit word), but it is padded with 4 zeros (12-bit slot) to satisfy the desired
protocol in interfacing to an external device. Within a slot, the bits may be shifted in/out of the McASP on the AXR[n]
pin either MSB or LSB first. When the word size is smaller than the slot size, the word may be aligned to the left
(beginning) of the slot or to the right (end) of the slot. The additional bits in the slot not belonging to the word may be
padded with 0, 1, or with one of the bits (the MSB or the LSB typically) from the data word. These options are shown
in
Figure 24-12. Definition of Bit, Word, and Slot
(1)
b7:b0 - bits. Bits b7 to b0 form a word.
(2)
P - pad bits. Bits b7 to b0, together with the four pad bits, form a slot.
(3)
In this example, the data is transmitted MSB first, left aligned.