UART Tx Block

Home > MPLAB Device Blocks for Simulink > Block Reference > Communication > UART > UART Tx Block

UART_Tx Block Icon The UART Tx block transmits data through a configured UART module. This block sends 8-bit data (uint8 type) and supports variable-length transmission with optional feedback on bytes sent. It works in conjunction with the UART Config block to provide flexible, efficient serial transmission.

Block Inputs/Outputs

Inputs:

Port Name	Data Type	Description	Optional
N	uint8/uint16	Number of bytes to send (variable length mode)	Yes (enable via parameter)
Tx	uint8 (vector or scalar)	Data to transmit	No (always present)
$	any	Function-Call input (triggers execution)	Yes (based on ordering)

Outputs:

Port Name	Data Type	Description	Optional
N	uint8/uint16	Number of bytes actually sent	Yes (enable via parameter)
$	function-call	Function-Call output (execution chain)	Yes (based on ordering)

Block Input/Output Datatype

The block accepts and outputs uint8 data values for UART transmission.

I/O Type	Range	Description
uint8 (input)	0 to 255	Raw byte data to transmit
uint8/uint16 (N output)	0 to buffer size	Number of bytes successfully sent

Supported Device Families

Family	Series	Architecture ID	Implementation Files
dsPIC	30F, 24F, 33F, 33E, 33C, 33A	1, 2, 3, 4, 8	PIC bare, Interrupt, DMA variants
PIC32	MK, MZ, MX, PIC32A	Included in dsPIC arch	Same as dsPIC (PIC implementations)
SAM E7x/S7x	SAME70, SAMS70, V71, PIC32CZ MC70	5	SAMx7 bare, Interrupt, DMA ping-pong
SAM E5x/D5x	SAME5x, SAMD5x	6	SAMx5 bare, Interrupt, DMA ping-pong
SAM C2x/D2x	SAMC2x, SAMD2x, SAMDA1	7	SAMx5 implementations (shared)

Configuration Parameters

UART Selection

UART Module - Select UART/USART number (must match UART Config block)
Dropdown automatically populated with available UARTs from selected chip

Data Transmission Options

Parameter	Options	Description
NSend Input	On/Off checkbox	Enable ‘N’ input port for variable-length transmission
NSend Output	On/Off checkbox	Enable ‘N’ output port (bytes actually sent)
UART String Mode	On/Off checkbox	When enabled: ‘0’ (null character) marks end of string
All or Nothing	On/Off checkbox	Only transmit if entire buffer can be sent (no partial sends)

Implementation Modes

Transmission behavior depends on the implementation mode selected in the UART Config block:

Mode ID	Implementation	Description	Thread Protection
1	Bare (Simplest)	Direct write to hardware FIFO (1/4/8 bytes)	Optional delay protection
2	Circular Buffer	Software buffer with interrupt-driven transmission	Critical section protection
3	DMA Ping-Pong	Two DMA buffers alternating (no data loss)	DMA channel management
4	DMA Single Buffer	Single DMA buffer (possible data loss if full)	DMA channel management
5	DMA Circular Buffer	DMA with circular buffer (same as mode 2)	Critical section protection

Thread Protection

The block provides configurable thread protection for multitasking environments:

Bit 0: Delay protection for tasks
Bit 1: Don’t send from tasks
Bit 2-3: Same for asynchronous interrupts
Common Values:
0x1: Delay task, don’t care interrupts
0x5: Delay all interrupts
0x9: Delay task, don’t send on interrupts
0xA: Don’t send (disabled)

Usage Examples

Basic Transmission (Fixed Length)

Add UART Config block (configure for UART 1, Circular Buffer Tx)
Add UART Tx block
Configure UART Tx:
UART Module: 1
NSend Input: Off
NSend Output: Off
UART String Mode: Off
All or Nothing: Off
Connect uint8 vector [5x1] to Tx input
Build and deploy

Variable Length String Transmission

Configure UART Tx block:
NSend Input: On
NSend Output: On (monitor bytes sent)
UART String Mode: On
All or Nothing: Off
Connect N input with dynamic length (e.g., from lookup table)
Connect Tx input with uint8 buffer [maxSize x 1]
Monitor N output to verify transmission

High-Reliability Transmission (DMA Ping-Pong)

UART Config: Select “DMA Ping-Pong Mode” for Tx
UART Tx: Enable “All or Nothing” mode
Connect fixed-size data packets
Result: No partial transmissions, guaranteed delivery

Implementation Details

Bare Mode (Mode 1)

Writes directly to hardware FIFO: U%TXREG = data[i]; /* Direct write to transmit register */

No buffering - immediate transmission
Limited to hardware FIFO size (1/4/8 bytes)
Fastest response, lowest overhead

Circular Buffer (Mode 2 & 5)

Software buffer with head/tail pointers
Interrupt-driven: TX interrupt sends next byte when ready
Buffer size configurable in UART Config block
Thread-safe with critical section protection

DMA Modes (Mode 3 & 4)

Ping-Pong: Two buffers alternate (buffer A → DMA → buffer B → DMA…)
Single Buffer: One buffer (faster but can lose data if full)
DMA channel automatically configured
Minimal CPU overhead during transmission

Error Handling

Missing UART Config Block

If no UART Config block is found for the selected UART:

Block display shows “MISSING UART X Config”
Warning printed to console
Compilation error: “UART TX is not configured”

Buffer Overflow

Circular Buffer: New data discarded if buffer full
DMA Single: Data lost if DMA busy
DMA Ping-Pong: Always accepts data (switches buffers)
All or Nothing: Entire packet rejected if insufficient space

[UART Config] - Configure UART module (required)
[UART Rx] - Receive data
[UART TxMatlab] - MATLAB interface for testing
[UART Brk/Autobaud] - Break detection and autobaud

Troubleshooting

Data Not Transmitting

Problem: UART Tx block present but no data on TX line.Solutions:

Verify UART Config block exists for same UART number
Check TX pin configuration in UART Config block
Verify UART enable (UARTEN bit)
Check baud rate matches receiver

Partial Data Loss

Problem: Some bytes missing from transmission.Solutions:

Increase buffer size in UART Config (Circular Buffer mode)
Switch to DMA Ping-Pong mode (no data loss)
Enable “All or Nothing” mode (reject partial sends)
Reduce transmission rate or increase UART baud rate

Multitasking Issues

Problem: Data corruption in multitasking model.Solutions:

Check thread protection settings in UART Config
Ensure only one task writes to same UART
Use DMA mode to avoid interrupt conflicts