AN2113 Freescale Semiconductor / Motorola, AN2113 Datasheet - Page 15

• Manufacturer Part Number: AN2113
• Description: AN2113 Multichannel Voice Coding System on the RTXC Operating System
• Manufacturer: Freescale Semiconductor / Motorola

7.2.2 Synchronous Interface
Dn = Audio Data for channel n
Cn = Control Data for channel n
ESSI0 transfers the data to and from the codecs. The ESSI0 interface has three main modes for handling
synchronous data transfers: normal, on-demand, and network. We use the network mode, which allows
multiple time slots of data to be transferred to the DSP within a given time frame. This mode is extremely
useful when an application requires the transfer of multiple independent channels of data through the same
interface. Network mode allows the ESSI to handle up to 32 time slots. This application uses two codecs,
which require a total of eight time slots per receive or transmit frame: four for audio data and four for
control data (see Figure 9).
Frame Sync
Data
Time Slots
The first four slots contain data from/to the first codec (two for data for left and right channels and two for
control information for left and right channels). The last four slots contain the data from/to the second
codec. Each audio channel is sampled at 8 KHz. In total, the ESSI0 handles 128 bits every 125 sec (16
bits per time slot for eight time slots every 8 KHz). The data received through ESSI0 is placed into a
16-element receive buffer, and the data transmitted is taken from a separate 16-element buffer.
A private (non-RTXC) ESSI receive ISR loads data/control information for each time-slot into the receive
buffer. Figure 10 shows the mechanism to receive data using the ESSI. The ESSI receive ISR simply reads
the audio data from the ESSI receive register and writes it into the 16-element receive buffer called
RX_buff.
Similarly, Figure 10 shows the mechanism used to transmit data using the ESSI. The ESSI private transmit
ISR simply writes the audio data from the 16-element receive buffer called TX_buff to the ESSI transmit
register.
3. Write to receive buffer
Figure 9. Picture of ESSI Network Mode Usage in This Application
1. ESSI Rx Event
D1
Freescale Semiconductor, Inc.
RX_buff
For More Information On This Product,
Figure 10. ESSI Data Receive Mechanism
D2
Multichannel Voice Coding System
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C1
ESSI Rx
ESSI
ISR
C2
2. Read audio data input
D3
D4
C3
Software Description
C4
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