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Part Number: AM2754-Q1
Other Parts Discussed in Thread: SYSCONFIG
While configuring McASP in sysconfig, I came across the mcasp slot bitmask option. What value should I select for this option? What is the use of this option? How the McASP operations change based on the transmit and receive slot bitmask?
Slot bitmasks in MCASP define which time slots in a TDM (Time Division Multiplexing) frame are active for transmitting or receiving data. Each bit in the bitmask corresponds to one time slot.
TDM Frame = Multiple Time Slots
Each Slot = One Audio Channel
Bitmask = Which slots are active (1) or inactive (0)
For a 16-slot TDM frame:
Bit Position: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Time Slot: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Bitmask: 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0
- Bit = 1: Slot is active (data transmitted/received)
- Bit = 0: Slot is inactive (no data)
---
EXAMPLE 1A: TDM-16 - TX SLOT MASK - SIMPLE 4-CHANNEL TX
Scenario: Use first 4 slots for transmission
TX Slot Mask:0x000F
Binary: 0000 0000 0000 1111
Active TX Slots: 0, 1, 2, 3
Visual:
Slots: | 0| 1| 2| 3| 4| 5| 6| 7| 8| 9|10|11|12|13|14|15|
TX: |TX|TX|TX|TX| -| -| -| -| -| -| -| -| -| -| -| -|
EXAMPLE 1B: TDM-16 - RX SLOT MASK - SIMPLE 4-CHANNEL RX
Scenario: Use slots 4-7 for reception
RX Slot Mask:0x00F0
Binary: 0000 0000 1111 0000
Active RX Slots: 4, 5, 6, 7
Visual:
Slots: | 0| 1| 2| 3| 4| 5| 6| 7| 8| 9|10|11|12|13|14|15|
RX: | -| -| -| -|RX|RX|RX|RX| -| -| -| -| -| -| -| -|
EXAMPLE 2A: TX SLOT MASK - STEREO AUDIO TX
Scenario: Stereo output transmission (2 channels)
TX Slot Mask: 3(or 0x3)
Binary: 11
Active TX Slots: 0, 1 (Left, Right)
Bitmask value range: 0-3 (only 2 bits needed)
Possible stereo TX bitmask values:
- 0(binary: 00) = No slots active
- 1(binary: 01) = Slot 0 active (Left only)
- 2(binary: 10) = Slot 1 active (Right only)
- 3(binary: 11) = Slots 0,1 active (Left + Right)
EXAMPLE 2B: RX SLOT MASK - STEREO AUDIO RX
Scenario: Stereo input reception (2 channels)
RX Slot Mask: 2(or 0x10)
Binary: 10
Active RX Slots: 1 (Left, Right)
Bitmask value range: 0-3 (only 2 bits needed)
Possible stereo RX bitmask values:
- 0(binary: 00) = No slots active
- 1(binary: 01) = Slot 0 active (Left only)
- 2(binary: 10) = Slot 1 active (Right only)
- 3(binary: 11) = Slots 0,1 active (Left + Right)
EXAMPLE 3A: TX SLOT MASK - SPARSE CHANNEL TX
Scenario: Non-contiguous slots for transmission
TX Slot Mask: 0xA5A5
Binary: 1010 0101 1010 0101
Active TX Slots: 0, 2, 5, 7, 8, 10, 13, 15
EXAMPLE 3B: RX SLOT MASK - SPARSE CHANNEL RX
Scenario: Non-contiguous slots for reception
RX Slot Mask: 0x5A5A
Binary: 0101 1010 0101 1010
Active RX Slots: 1, 3, 4, 6, 9, 11, 12, 14
BASIC PROCESS:
When you set a receive bitmask, the MCASP hardware:
DATA FLOW PROCESS:
Step 1: Incoming TDM Frame
Incoming TDM Frame (16 slots):
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |10 |11 |12 |13 |14 |15 |
| A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Note: Each slot always contains the same type of data (A=Slot0, B=Slot1, etc.).
The subscript number indicates the frame number.
Step 2: Bitmask Filtering
The receive bitmask determines which slots get captured.
Step 3: Buffer Filling
Active slots are stored sequentially in the receive buffer.
EXAMPLES:
EXAMPLE 1: SIMPLE 4-SLOT RECEPTION
RX Bitmask: 0x000F(slots 0, 1, 2, 3 active)
TDM Frame Structure (Every Frame):
Slots: | 0| 1| 2| 3| 4| 5| 6| 7| 8| 9|10|11|12|13|14|15|
Data: | A| B| C| D| E| F| G| H| I| J| K| L| M| N| O| P|
Active:| Y| Y| Y| Y| N| N| N| N| N| N| N| N| N| N| N| N|
Receive Buffer Content (Multiple Frames):
Buffer Pos: [0] [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Frame 1: A1 B1 C1 D1
Frame 2: A2 B2 C2 D2
Frame 3: A3 B3 C3 D3
Frame 4: A4 B4 C4 D4
Frame 5: A5 B5
Source Slot: 0 1 2 3 0 1 2 3 0 1 2 3 0 1 2 3 0 1
Explanation:
- A1= Slot 0 data from Frame 1
- B2= Slot 1 data from Frame 2
- C3= Slot 2 data from Frame 3, etc.
EXAMPLE 2: NON-CONTIGUOUS SLOTS
RX Bitmask: 0x5050 (slots 4, 6, 12, 14 active)
TDM Frame Structure (Every Frame):
Slots: | 0| 1| 2| 3| 4| 5| 6| 7| 8| 9|10|11|12|13|14|15|
Data: | A| B| C| D| E| F| G| H| I| J| K| L| M| N| O| P|
Active:|N |N | N| N| Y| N| Y|N| N| N| N| N| Y| N| Y| N|
Receive Buffer Content (Multiple Frames):
Buffer Pos: [0] [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Frame 1: E1 G1 M1 O1
Frame 2: E2 G2 M2 O2
Frame 3: E3 G3 M3 O3
Frame 4: E4 G4 M4 O4
Frame 5: E5 G5
Source Slot: 4 6 12 14 4 6 12 14 4 6 12 14 4 6 12 14 4 6
Explanation:
- E1= Slot 4 data from Frame 1
- G2= Slot 6 data from Frame 2
- M3= Slot 12 data from Frame 3
- O4= Slot 14 data from Frame 4, etc.
