Introduction

EnDat3 is the latest generation of the bidirectional serial interface for position encoders, providing enhanced safety features and higher data rates compared to EnDat 2.2. During EnDat3 operation, the EnDat receiver (subsequent electronics) communicates with EnDat3 position encoders to obtain position information and additional diagnostic data.

EnDat3 introduces several key improvements over previous versions:

Higher data transfer rates (up to 25 Mbps)
Enhanced safety features with frame-based CRC protection
Improved error detection and recovery mechanisms
Support for both host-triggered and periodic trigger modes
Background and foreground communication channels for concurrent operations

Note: This implementation uses the PRU-ICSS peripheral interface mode. Refer to Peripheral IF mode for more details.

Features Supported

EnDat3 protocol specification compliance
Data transfer rates: 12.5 Mbps (25 Mbps not currently supported)
Single channel operation
Three modes of operation:
- Host trigger mode: Commands initiated by host controller
- Periodic CMP trigger mode: Automatic position updates triggered by IEP compare events
- Periodic CAP trigger mode: Automatic position updates triggered by IEP capture events
Frame-based protocol with three frame types:
- High Priority Frame (HPF): Position data and critical status
- Low Priority Header (LPH): Information about following LPF frames
- Low Priority Frame (LPF): Additional data and diagnostics
Foreground communication commands:
- DATA0-DATA7: Activate LPF send lists
- DATA: General data with background data
- DATANOP: Data without background data
- RESET: Encoder reset
- CLEAR: Reset encoder states
- ECHO: Propagation time measurement
- RATE: Set data transfer rate
- HELLO: Switch to EnDat3 mode
- FORCE: Forced dynamic sampling
- BUSBC: Bus broadcast command
- BUSP2P: Bus point-to-point command
- BUSINIT: Bus initialization
Background communication commands:
- NOP: No operation
- READ: Read from encoder memory
- WRITE: Write to encoder memory
- RECONFIGURE: Reconfigure parameters
- AUTH: Authentication with user levels
- PROTECT: Set memory protection
- SETPASS: Set password for user level
- LOCATE: Encoder location function
Continuous position fetch mode for real-time applications
Automatic CRC verification for all frame types
Error detection and reporting with detailed error codes
Manchester encoding support for robust data transmission
Flexible channel configuration
- Supported on PRU-ICSSM
- PRU0 or PRU1 configuration via SysConfig

Features Not Supported

In general, peripherals or features not mentioned as part of "Features Supported" section are not supported in this release, including the below:

25 Mbps Data Rate: Currently only 12.5 Mbps is supported and tested
Multi-channel concurrent operation: Only single channel is supported
Daisy Chain Topology: Daisy chaining multiple encoders is not supported
Cable length up to 100m

Limitations

Cable delays must be compensated for frequencies above 10 MHz
Recovery time settings must match encoder specifications
HELLO command may take up to 300ms to complete encoder initialization
RESET command may take up to 300ms for encoder restart
RATE command switching occurs after 2ms of no communication and may take up to 300ms

SysConfig Features

Note: It is strongly recommend to use SysConfig where it is available instead of using direct SW API calls. This will help simplify the SW application and also catch common mistakes early in the development cycle.

Attention: One module instance should be created in SysConfig per PRU-ICSS slice being used for EnDat3.

SysConfig can be used to configure the following:

Selecting the ICSSM PRU slice (Tested on ICSSM-PRU0)
Configuring PINMUX
Channel selection (Channel 0, Channel 1, or Channel 2)
Selecting baud rate (12.5 Mbps)
IEP event selection for periodic trigger mode
Booster Pack Support: Enable when using BP-AM2BLDCSERVO

Note: EnDAT3 firmware is tested with ICSS Core Clock running at 200 MHz frequency only due to clock divider requirements.

PRU-ICSS Resource Usage

Utilizes the Peripheral IF mode (3-channel peripheral interface mode) for EnDAT3 communication. Maximum of 3 channels are available per PRU slice. (Refer Peripheral IF mode for more details)
Each channel has 4 pins (Clock, Data out, Data in, Output enable)
Following table contains details of memory usage, IEP usage and interrupt controller usage:

Attention: In addition to the following resources used by PRU firmware, SDK examples also configure IEP0 CMP0 for IEP counter reset in periodic trigger CMP mode.

Configuration per slice	PRU Core	Memory Usage	IEP Usage	Interrupt Controller (INTC) Usage	Description
Single channel	PRUx	DMEM: 508 Bytes (0x0000 to 0x01FB) IMEM: ~ 3.55 kB	CMP Mode: IEP0 CMPy for trigger (IEP0 CMPy selected in SysConfig) CAP Mode: IEP0 CAPy for trigger (IEP0 CAPy selected in SysConfig)	INTC event/input number 18 or 21 (prx_pru_mst_intr[2/5]_intr_req) is used to trigger interrupt to Arm® Cortex®-R5F based on slice	IEP, CMP/CAP events and INTC signals are used only in periodic trigger modes

Note: For pin usage, see Pin Multiplexing section.

Interface Clock Frequencies

The EnDat3 interface supports two standard data transfer rates:

Data Rate	TX Clock Frequency	RX Clock Frequency	Note
12.5 Mbps	25 MHz	100 MHz (8x oversampling)	Supported in this release
25 Mbps	50 MHz	200 MHz (8x oversampling)	Not supported in this release

PRU Core Clock Requirements

The PRU core clock must be configured to support the required TX and RX clock frequencies:

PRU Core Clock	Supported Data Rates
200 MHz	12.5 Mbps

EnDat3 Protocol Overview

Frame Structure

EnDat3 uses a frame-based protocol with three types of frames:

High Priority Frame (HPF)
- Contains position data (48 bits)
- Status byte with error/warning flags
- CRC-8 checksum
- Transmitted in every communication cycle
Low Priority Header (LPH)
- Indicates number of following LPF frames
- Communication status information
- CRC-8 checksum
Low Priority Frame (LPF)
- Additional data and diagnostics (48 bits per frame)
- Frame ID (FID) in status byte
- CRC-8 checksum
- Up to 8 frames can follow HPF and LPH

Communication Flow

Host sends command via foreground channel
Encoder responds with HPF (position + status)
If background data requested, encoder sends LPH
Encoder sends configured number of LPF frames
All frames are CRC-verified by receiver

Operating Modes

Host Trigger Mode

Application controls command timing
Suitable for R5F driven position updates

Periodic CMP Trigger Mode

IEP timer compare event triggers position sampling
Compare events occur when the IEP timer counter matches the configured compare value
Enables fixed-rate periodic sampling

Configuration:

Compare event range: CMP0-CMP15 (0-15)
Mode configured via endat3_set_operating_mode() API
IEP compare event number set via endat3_config_iep_cmp_event() API
Event selection can be done in SysConfig
IEP configuration and CMP event configuration should be done in application. Driver uses above APIs to inform firmware to enable CMP periodic mode and uses the configured CMP event to start sampling periodically.

Periodic CAP Trigger Mode

External signals trigger position sampling through IEP capture events
The capture event is triggered on the rising edge of the external input pulse, enabling event-driven position capture
Internal signals can also be mapped to IEP capture events via XBAR