DCM - Diagnostic Communication Manager - AUTOSAR Classic BSW

DSL Layer: Session Management & Timing

DSL Timer	Parameter	Purpose
S3Server	DcmDslProtocolS3Server (default 5000 ms)	Non-default session timeout — returns to Default session if no request within S3
P2Server	DcmTimStrP2ServerMax (default 50 ms)	Max time to start sending response after receiving request
P2*Server	DcmTimStrP2StarServerMax (default 5000 ms)	Extended response time after NRC 0x78 (pending)

XMLDcm_Cfg.arxml

<DCM-DSL-PROTOCOL-ROW>
  <DCM-PROTOCOL-TIME-LIMIT>
    <DCM-DSL-PROTOCOL-TRANS-LAYER-TIMING>
      <DCM-P2-TIMING>50</DCM-P2-TIMING>
      <DCM-P2-STAR-TIMING>5000</DCM-P2-STAR-TIMING>
    </DCM-DSL-PROTOCOL-TRANS-LAYER-TIMING>
  </DCM-PROTOCOL-TIME-LIMIT>
</DCM-DSL-PROTOCOL-ROW>

DSD Layer: Service Dispatching

Parameter	Description
DcmDsdServiceTable	Table of supported UDS services — service ID + session/security conditions
DcmDsdService	One entry per service (0x10, 0x19, 0x22, 0x27, 0x2E, 0x31, 0x34–0x37)
DcmDsdSubService	Sub-function entries (e.g., 0x10.01 = Default, 0x10.03 = Extended)
DcmDsdServiceSessionModeRuleRef	Which sessions are allowed to use this service
DcmDsdServiceSecurityLevelRef	Which security levels are required for this service

DSP Layer: DID Configuration

CApp_ReadVehicleSpeed.c

/* DcmDspDidReadDataFnc callback */
FUNC(Std_ReturnType, DCM_APPL_CODE) App_ReadVehicleSpeed(
    P2VAR(uint8, AUTOMATIC, DCM_APPL_DATA) Data)
{
    uint16 speedRaw;
    (void)Rte_Read_DiagSWC_SpeedIn_VehicleSpeed(&speedRaw);
    Data[0] = (uint8)(speedRaw >> 8);
    Data[1] = (uint8)(speedRaw & 0xFF);
    return E_OK;
}

SecurityAccess 0x27: Seed/Key

CApp_SecurityAccess.c

FUNC(Std_ReturnType, DCM_APPL_CODE) App_GetSeed(
    P2CONST(Dcm_SecLevelType, AUTOMATIC, DCM_APPL_CONST) SecurityLevel,
    P2CONST(uint8, AUTOMATIC, DCM_APPL_DATA) SeedLen,
    P2VAR(uint8, AUTOMATIC, DCM_APPL_DATA) Seed,
    P2VAR(uint8, AUTOMATIC, DCM_APPL_DATA) ErrorCode)
{
    static uint32 storedSeed;
    storedSeed = GetCryptoRandomSeed(); /* Hardware RNG or TRNG */
    Seed[0] = (uint8)(storedSeed >> 24);
    Seed[1] = (uint8)(storedSeed >> 16);
    Seed[2] = (uint8)(storedSeed >> 8);
    Seed[3] = (uint8)(storedSeed);
    return E_OK;
}

FUNC(Std_ReturnType, DCM_APPL_CODE) App_CompareKey(
    P2CONST(uint8, AUTOMATIC, DCM_APPL_DATA) Key,
    P2CONST(Dcm_SecLevelType, AUTOMATIC, DCM_APPL_CONST) SecurityLevel,
    P2VAR(uint8, AUTOMATIC, DCM_APPL_DATA) ErrorCode)
{
    uint32 rx = ((uint32)Key[0]<<24)|((uint32)Key[1]<<16)|
                ((uint32)Key[2]<<8)|(uint32)Key[3];
    if (rx == DeriveExpectedKey(storedSeed)) return E_OK;
    *ErrorCode = DCM_E_REQUIREDTIMEDELAYNOTEXPIRED;
    return E_NOT_OK; /* NRC 0x35 */
}

Summary

DCM configuration requires precise session/security matrix definition in DcmDsdServiceTable. Every DID needs a corresponding read/write callback. SecurityAccess seed/key callbacks must use a cryptographically strong RNG — a predictable seed is a vehicle security vulnerability.

🔬 Deep Dive — Core Concepts Expanded

This section builds on the foundational concepts covered above with additional technical depth, edge cases, and configuration nuances that separate competent engineers from experts. When working on production ECU projects, the details covered here are the ones most commonly responsible for integration delays and late-phase defects.

Key principles to reinforce:

Configuration over coding: In AUTOSAR and automotive middleware environments, correctness is largely determined by ARXML configuration, not application code. A correctly implemented algorithm can produce wrong results due to a single misconfigured parameter.
Traceability as a first-class concern: Every configuration decision should be traceable to a requirement, safety goal, or architecture decision. Undocumented configuration choices are a common source of regression defects when ECUs are updated.
Cross-module dependencies: In tightly integrated automotive software stacks, changing one module's configuration often requires corresponding updates in dependent modules. Always perform a dependency impact analysis before submitting configuration changes.

🏭 How This Topic Appears in Production Projects

Project integration phase: The concepts covered in this lesson are most commonly encountered during ECU integration testing — when multiple software components from different teams are combined for the first time. Issues that were invisible in unit tests frequently surface at this stage.
Supplier/OEM interface: This is a topic that frequently appears in technical discussions between Tier-1 ECU suppliers and OEM system integrators. Engineers who can speak fluently about these details earn credibility and are often brought into critical design review meetings.
Automotive tool ecosystem: Vector CANoe/CANalyzer, dSPACE tools, and ETAS INCA are the standard tools used to validate and measure the correct behaviour of the systems described in this lesson. Familiarity with these tools alongside the conceptual knowledge dramatically accelerates debugging in real projects.

⚠️ Common Mistakes and How to Avoid Them

Assuming default configuration is correct: Automotive software tools ship with default configurations that are designed to compile and link, not to meet project-specific requirements. Every configuration parameter needs to be consciously set. 'It compiled' is not the same as 'it is correctly configured'.
Skipping documentation of configuration rationale: In a 3-year ECU project with team turnover, undocumented configuration choices become tribal knowledge that disappears when engineers leave. Document why a parameter is set to a specific value, not just what it is set to.
Testing only the happy path: Automotive ECUs must behave correctly under fault conditions, voltage variations, and communication errors. Always test the error handling paths as rigorously as the nominal operation. Many production escapes originate in untested error branches.
Version mismatches between teams: In a multi-team project, the BSW team, SWC team, and system integration team may use different versions of the same ARXML file. Version management of all ARXML files in a shared repository is mandatory, not optional.

📊 Industry Note

Engineers who master both the theoretical concepts and the practical toolchain skills covered in this course are among the most sought-after professionals in the automotive software industry. The combination of AUTOSAR standards knowledge, safety engineering understanding, and hands-on configuration experience commands premium salaries at OEMs and Tier-1 suppliers globally.