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XRSR
XRBUF
32
32
32
SRCTL
Serializer
Control
Pin
function
control
AXR[n] Pin
Transmit
format unit
Receive
format unit
1096
SPRUH82C – April 2013 – Revised September 2016
Copyright © 2013–2016, Texas Instruments Incorporated
Multichannel Audio Serial Port (McASP)
24.0.21.4 Clocking Examples
Some examples of processes using the McASP clocking and frame flexibility are:
•
Receive data from a DVD at 48 kHz, but output up-sampled or decoded audio at 96 kHz or 192 kHz.
This could be accomplished by inputting a high-frequency master clock (for example, 512 ×
receive FS), receiving with an internally-generated bit clock ratio of divide-by-8, and transmitting with
an internally-generated bit clock ratio of divide-by-4 or divide-by-2.
•
Transmit/receive data based on one sample rate (for example, 44.1 kHz), and transmit/receive data at
a different sample rate (for example, 48 kHz).
General Architecture
24.0.21.1 Serializers
The serializers take care of shifting serial data in and out of the McASP. Each serializer consists of a shift
register (XRSR), a data buffer (XRBUF), a control register (SRCTL), and logic to support the data
alignment options of the McASP. For each serializer, there is a dedicated serial data pin (AXR[n]) and a
dedicated control register (SRCTL[n]). The control register allows the serializer to be configured as a
transmitter, receiver, or as inactive. When configured as a transmitter the serializer shifts out data to the
serial data pin AXR[n]. When configured as a receiver, the serializer shifts in data from the AXR[n] pin.
The serializer is clocked from the transmit/receive section clock (ACLKX/ACLKR) if configured to
transmit/receive respectively.
All serializers that are configured to transmit operate in lock-step. Similarly, all serializers that are
configured to receive also operate in lock-step. This means that at most there are two zones per McASP,
one for transmit and one for receive.
shows the block diagram of the serializer and its interface to other units within the McASP.
Figure 24-18. Individual Serializer and Connections Within McASP
For receive, data is shifted in through the AXR[n] pin to the shift register XRSR. Once the entire slot of
data is collected in the XRSR, the data is copied to the data buffer XRBUF. The data is now ready to be
read by the CPU through the RBUF register, which is an alias of the XRBUF for receive. When the CPU
reads from the RBUF, the McASP passes the data from RBUF through the receive format unit and returns
the formatted data to the CPU.
For transmit, the CPU services the McASP by writing data into the XBUF register, which is an alias of the
XRBUF for transmit. The data automatically passes through the transmit format unit before actually
reaching the XRBUF in the serializer. The data is then copied from XRBUF to XRSR, and shifted out from
the AXR[n] synchronously to the serial clock.
In DIT mode, in addition to the data, the serializer shifts out other DIT-specific information accordingly
(preamble, user data, etc.).
The serializer configuration is controlled by SRCTL[n].