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JTAG Debug Interface

LibXR::Debug::Jtag defines TAP state control and IR/DR shifting. Current implementations include the GPIO bit-bang backend JtagGeneralGPIO and the DP/AP transaction layer JtagDp.

Jtag base class​

virtual ErrorCode SetClockHz(uint32_t hz) = 0;
virtual void Close() = 0;
virtual ErrorCode ResetTap() = 0;
virtual ErrorCode GotoState(TapState target) = 0;
virtual ErrorCode ShiftIR(uint32_t bits,
                          const uint8_t* in_lsb_first,
                          uint8_t* out_lsb_first) = 0;
virtual ErrorCode ShiftDR(uint32_t bits,
                          const uint8_t* in_lsb_first,
                          uint8_t* out_lsb_first) = 0;
virtual ErrorCode Sequence(uint32_t cycles, bool tms,
                           const uint8_t* tdi_lsb_first,
                           uint8_t* tdo_lsb_first) = 0;
virtual void IdleClocks(uint32_t cycles) = 0;

IR/DR data is LSB-first. A null input shifts zeroes; a null output discards TDO.

GPIO backend​

#include "debug/jtag_general_gpio.hpp"

LibXR::Debug::JtagGeneralGPIO<decltype(tck), decltype(tms),
                            decltype(tdi), decltype(tdo)>
    jtag(tck, tms, tdi, tdo, loops_per_us, 500000);

Construction configures TCK/TMS/TDI/TDO and resets TAP. loops_per_us calibrates software delay; target TCK is set through SetClockHz().

The GPIO GotoState() directly handles RESET, IDLE, SHIFT_IR and SHIFT_DR. ShiftIR() and ShiftDR() return to IDLE when complete.

JTAG-DP​

#include "debug/jtag_dp.hpp"

LibXR::Debug::JtagDp dp(jtag);
uint32_t idcode = 0;
auto result = dp.ReadIdCode(idcode);

DpReadTxn() and ApReadTxn() handle pipelined JTAG-DP reads by fetching the result from RDBUFF after the read request. Low-level DpRead() / DpWrite() perform one DP transaction directly.

Multi-device chains use JtagProtocol::ChainConfig to specify device count, selected index and IR lengths. Call SetChainConfig() when chain configuration changes.

CMSIS-DAP​

CMSIS-DAP v1/v2 JTAG profiles bind the same backend through SetJtag(). See DAP v1 and DAP v2 for profile and USB endpoint configuration.