src/jtag/drivers/bitbang.c

   1 /***************************************************************************
   2  *   Copyright (C) 2005 by Dominic Rath                                    *
   3  *   Dominic.Rath@gmx.de                                                   *
   4  *                                                                         *
   5  *   Copyright (C) 2007,2008 Øyvind Harboe                                 *
   6  *   oyvind.harboe@zylin.com                                               *
   7  *                                                                         *
   8  *   This program is free software; you can redistribute it and/or modify  *
   9  *   it under the terms of the GNU General Public License as published by  *
  10  *   the Free Software Foundation; either version 2 of the License, or     *
  11  *   (at your option) any later version.                                   *
  12  *                                                                         *
  13  *   This program is distributed in the hope that it will be useful,       *
  14  *   but WITHOUT ANY WARRANTY; without even the implied warranty of        *
  15  *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the         *
  16  *   GNU General Public License for more details.                          *
  17  *                                                                         *
  18  *   You should have received a copy of the GNU General Public License     *
  19  *   along with this program.  If not, see <http://www.gnu.org/licenses/>. *
  20  ***************************************************************************/
  21
  22 /* 2014-12: Addition of the SWD protocol support is based on the initial work
  23  * by Paul Fertser and modifications by Jean-Christian de Rivaz. */
  24
  25 #ifdef HAVE_CONFIG_H
  26 #include "config.h"
  27 #endif
  28
  29 #include "bitbang.h"
  30 #include <jtag/interface.h>
  31 #include <jtag/commands.h>
  32
  33 /**
  34  * Function bitbang_stableclocks
  35  * issues a number of clock cycles while staying in a stable state.
  36  * Because the TMS value required to stay in the RESET state is a 1, whereas
  37  * the TMS value required to stay in any of the other stable states is a 0,
  38  * this function checks the current stable state to decide on the value of TMS
  39  * to use.
  40  */
  41 static int bitbang_stableclocks(int num_cycles);
  42
  43 static void bitbang_swd_write_reg(uint8_t cmd, uint32_t value, uint32_t ap_delay_clk);
  44
  45 struct bitbang_interface *bitbang_interface;
  46
  47 /* DANGER!!!! clock absolutely *MUST* be 0 in idle or reset won't work!
  48  *
  49  * Set this to 1 and str912 reset halt will fail.
  50  *
  51  * If someone can submit a patch with an explanation it will be greatly
  52  * appreciated, but as far as I can tell (ØH) DCLK is generated upon
  53  * clk = 0 in TAP_IDLE. Good luck deducing that from the ARM documentation!
  54  * The ARM documentation uses the term "DCLK is asserted while in the TAP_IDLE
  55  * state". With hardware there is no such thing as *while* in a state. There
  56  * are only edges. So clk => 0 is in fact a very subtle state transition that
  57  * happens *while* in the TAP_IDLE state. "#&¤"#¤&"#&"#&
  58  *
  59  * For "reset halt" the last thing that happens before srst is asserted
  60  * is that the breakpoint is set up. If DCLK is not wiggled one last
  61  * time before the reset, then the breakpoint is not set up and
  62  * "reset halt" will fail to halt.
  63  *
  64  */
  65 #define CLOCK_IDLE() 0
  66
  67 /* The bitbang driver leaves the TCK 0 when in idle */
  68 static void bitbang_end_state(tap_state_t state)
  69 {
  70         assert(tap_is_state_stable(state));
  71         tap_set_end_state(state);
  72 }
  73
  74 static int bitbang_state_move(int skip)
  75 {
  76         int i = 0, tms = 0;
  77         uint8_t tms_scan = tap_get_tms_path(tap_get_state(), tap_get_end_state());
  78         int tms_count = tap_get_tms_path_len(tap_get_state(), tap_get_end_state());
  79
  80         for (i = skip; i < tms_count; i++) {
  81                 tms = (tms_scan >> i) & 1;
  82                 if (bitbang_interface->write(0, tms, 0) != ERROR_OK)
  83                         return ERROR_FAIL;
  84                 if (bitbang_interface->write(1, tms, 0) != ERROR_OK)
  85                         return ERROR_FAIL;
  86         }
  87         if (bitbang_interface->write(CLOCK_IDLE(), tms, 0) != ERROR_OK)
  88                 return ERROR_FAIL;
  89
  90         tap_set_state(tap_get_end_state());
  91         return ERROR_OK;
  92 }
  93
  94 /**
  95  * Clock a bunch of TMS (or SWDIO) transitions, to change the JTAG
  96  * (or SWD) state machine.
  97  */
  98 static int bitbang_execute_tms(struct jtag_command *cmd)
  99 {
 100         unsigned num_bits = cmd->cmd.tms->num_bits;
 101         const uint8_t *bits = cmd->cmd.tms->bits;
 102
 103         LOG_DEBUG_IO("TMS: %d bits", num_bits);
 104
 105         int tms = 0;
 106         for (unsigned i = 0; i < num_bits; i++) {
 107                 tms = ((bits[i/8] >> (i % 8)) & 1);
 108                 if (bitbang_interface->write(0, tms, 0) != ERROR_OK)
 109                         return ERROR_FAIL;
 110                 if (bitbang_interface->write(1, tms, 0) != ERROR_OK)
 111                         return ERROR_FAIL;
 112         }
 113         if (bitbang_interface->write(CLOCK_IDLE(), tms, 0) != ERROR_OK)
 114                 return ERROR_FAIL;
 115
 116         return ERROR_OK;
 117 }
 118
 119 static int bitbang_path_move(struct pathmove_command *cmd)
 120 {
 121         int num_states = cmd->num_states;
 122         int state_count;
 123         int tms = 0;
 124
 125         state_count = 0;
 126         while (num_states) {
 127                 if (tap_state_transition(tap_get_state(), false) == cmd->path[state_count])
 128                         tms = 0;
 129                 else if (tap_state_transition(tap_get_state(), true) == cmd->path[state_count])
 130                         tms = 1;
 131                 else {
 132                         LOG_ERROR("BUG: %s -> %s isn't a valid TAP transition",
 133                                 tap_state_name(tap_get_state()),
 134                                 tap_state_name(cmd->path[state_count]));
 135                         exit(-1);
 136                 }
 137
 138                 if (bitbang_interface->write(0, tms, 0) != ERROR_OK)
 139                         return ERROR_FAIL;
 140                 if (bitbang_interface->write(1, tms, 0) != ERROR_OK)
 141                         return ERROR_FAIL;
 142
 143                 tap_set_state(cmd->path[state_count]);
 144                 state_count++;
 145                 num_states--;
 146         }
 147
 148         if (bitbang_interface->write(CLOCK_IDLE(), tms, 0) != ERROR_OK)
 149                 return ERROR_FAIL;
 150
 151         tap_set_end_state(tap_get_state());
 152         return ERROR_OK;
 153 }
 154
 155 static int bitbang_runtest(int num_cycles)
 156 {
 157         int i;
 158
 159         tap_state_t saved_end_state = tap_get_end_state();
 160
 161         /* only do a state_move when we're not already in IDLE */
 162         if (tap_get_state() != TAP_IDLE) {
 163                 bitbang_end_state(TAP_IDLE);
 164                 if (bitbang_state_move(0) != ERROR_OK)
 165                         return ERROR_FAIL;
 166         }
 167
 168         /* execute num_cycles */
 169         for (i = 0; i < num_cycles; i++) {
 170                 if (bitbang_interface->write(0, 0, 0) != ERROR_OK)
 171                         return ERROR_FAIL;
 172                 if (bitbang_interface->write(1, 0, 0) != ERROR_OK)
 173                         return ERROR_FAIL;
 174         }
 175         if (bitbang_interface->write(CLOCK_IDLE(), 0, 0) != ERROR_OK)
 176                 return ERROR_FAIL;
 177
 178         /* finish in end_state */
 179         bitbang_end_state(saved_end_state);
 180         if (tap_get_state() != tap_get_end_state())
 181                 if (bitbang_state_move(0) != ERROR_OK)
 182                         return ERROR_FAIL;
 183
 184         return ERROR_OK;
 185 }
 186
 187 static int bitbang_stableclocks(int num_cycles)
 188 {
 189         int tms = (tap_get_state() == TAP_RESET ? 1 : 0);
 190         int i;
 191
 192         /* send num_cycles clocks onto the cable */
 193         for (i = 0; i < num_cycles; i++) {
 194                 if (bitbang_interface->write(1, tms, 0) != ERROR_OK)
 195                         return ERROR_FAIL;
 196                 if (bitbang_interface->write(0, tms, 0) != ERROR_OK)
 197                         return ERROR_FAIL;
 198         }
 199
 200         return ERROR_OK;
 201 }
 202
 203 static int bitbang_scan(bool ir_scan, enum scan_type type, uint8_t *buffer,
 204                 unsigned scan_size)
 205 {
 206         tap_state_t saved_end_state = tap_get_end_state();
 207         unsigned bit_cnt;
 208
 209         if (!((!ir_scan &&
 210                         (tap_get_state() == TAP_DRSHIFT)) ||
 211                         (ir_scan && (tap_get_state() == TAP_IRSHIFT)))) {
 212                 if (ir_scan)
 213                         bitbang_end_state(TAP_IRSHIFT);
 214                 else
 215                         bitbang_end_state(TAP_DRSHIFT);
 216
 217                 if (bitbang_state_move(0) != ERROR_OK)
 218                         return ERROR_FAIL;
 219                 bitbang_end_state(saved_end_state);
 220         }
 221
 222         size_t buffered = 0;
 223         for (bit_cnt = 0; bit_cnt < scan_size; bit_cnt++) {
 224                 int tms = (bit_cnt == scan_size-1) ? 1 : 0;
 225                 int tdi;
 226                 int bytec = bit_cnt/8;
 227                 int bcval = 1 << (bit_cnt % 8);
 228
 229                 /* if we're just reading the scan, but don't care about the output
 230                  * default to outputting 'low', this also makes valgrind traces more readable,
 231                  * as it removes the dependency on an uninitialised value
 232                  */
 233                 tdi = 0;
 234                 if ((type != SCAN_IN) && (buffer[bytec] & bcval))
 235                         tdi = 1;
 236
 237                 if (bitbang_interface->write(0, tms, tdi) != ERROR_OK)
 238                         return ERROR_FAIL;
 239
 240                 if (type != SCAN_OUT) {
 241                         if (bitbang_interface->buf_size) {
 242                                 if (bitbang_interface->sample() != ERROR_OK)
 243                                         return ERROR_FAIL;
 244                                 buffered++;
 245                         } else {
 246                                 switch (bitbang_interface->read()) {
 247                                         case BB_LOW:
 248                                                 buffer[bytec] &= ~bcval;
 249                                                 break;
 250                                         case BB_HIGH:
 251                                                 buffer[bytec] |= bcval;
 252                                                 break;
 253                                         default:
 254                                                 return ERROR_FAIL;
 255                                 }
 256                         }
 257                 }
 258
 259                 if (bitbang_interface->write(1, tms, tdi) != ERROR_OK)
 260                         return ERROR_FAIL;
 261
 262                 if (type != SCAN_OUT && bitbang_interface->buf_size &&
 263                                 (buffered == bitbang_interface->buf_size ||
 264                                  bit_cnt == scan_size - 1)) {
 265                         for (unsigned i = bit_cnt + 1 - buffered; i <= bit_cnt; i++) {
 266                                 switch (bitbang_interface->read_sample()) {
 267                                         case BB_LOW:
 268                                                 buffer[i/8] &= ~(1 << (i % 8));
 269                                                 break;
 270                                         case BB_HIGH:
 271                                                 buffer[i/8] |= 1 << (i % 8);
 272                                                 break;
 273                                         default:
 274                                                 return ERROR_FAIL;
 275                                 }
 276                         }
 277                         buffered = 0;
 278                 }
 279         }
 280
 281         if (tap_get_state() != tap_get_end_state()) {
 282                 /* we *KNOW* the above loop transitioned out of
 283                  * the shift state, so we skip the first state
 284                  * and move directly to the end state.
 285                  */
 286                 if (bitbang_state_move(1) != ERROR_OK)
 287                         return ERROR_FAIL;
 288         }
 289         return ERROR_OK;
 290 }
 291
 292 int bitbang_execute_queue(void)
 293 {
 294         struct jtag_command *cmd = jtag_command_queue;  /* currently processed command */
 295         int scan_size;
 296         enum scan_type type;
 297         uint8_t *buffer;
 298         int retval;
 299
 300         if (!bitbang_interface) {
 301                 LOG_ERROR("BUG: Bitbang interface called, but not yet initialized");
 302                 exit(-1);
 303         }
 304
 305         /* return ERROR_OK, unless a jtag_read_buffer returns a failed check
 306          * that wasn't handled by a caller-provided error handler
 307          */
 308         retval = ERROR_OK;
 309
 310         if (bitbang_interface->blink) {
 311                 if (bitbang_interface->blink(1) != ERROR_OK)
 312                         return ERROR_FAIL;
 313         }
 314
 315         while (cmd) {
 316                 switch (cmd->type) {
 317                         case JTAG_RUNTEST:
 318                                 LOG_DEBUG_IO("runtest %i cycles, end in %s",
 319                                                 cmd->cmd.runtest->num_cycles,
 320                                                 tap_state_name(cmd->cmd.runtest->end_state));
 321                                 bitbang_end_state(cmd->cmd.runtest->end_state);
 322                                 if (bitbang_runtest(cmd->cmd.runtest->num_cycles) != ERROR_OK)
 323                                         return ERROR_FAIL;
 324                                 break;
 325
 326                         case JTAG_STABLECLOCKS:
 327                                 /* this is only allowed while in a stable state.  A check for a stable
 328                                  * state was done in jtag_add_clocks()
 329                                  */
 330                                 if (bitbang_stableclocks(cmd->cmd.stableclocks->num_cycles) != ERROR_OK)
 331                                         return ERROR_FAIL;
 332                                 break;
 333
 334                         case JTAG_TLR_RESET:
 335                                 LOG_DEBUG_IO("statemove end in %s",
 336                                                 tap_state_name(cmd->cmd.statemove->end_state));
 337                                 bitbang_end_state(cmd->cmd.statemove->end_state);
 338                                 if (bitbang_state_move(0) != ERROR_OK)
 339                                         return ERROR_FAIL;
 340                                 break;
 341                         case JTAG_PATHMOVE:
 342                                 LOG_DEBUG_IO("pathmove: %i states, end in %s",
 343                                                 cmd->cmd.pathmove->num_states,
 344                                                 tap_state_name(cmd->cmd.pathmove->path[cmd->cmd.pathmove->num_states - 1]));
 345                                 if (bitbang_path_move(cmd->cmd.pathmove) != ERROR_OK)
 346                                         return ERROR_FAIL;
 347                                 break;
 348                         case JTAG_SCAN:
 349                                 bitbang_end_state(cmd->cmd.scan->end_state);
 350                                 scan_size = jtag_build_buffer(cmd->cmd.scan, &buffer);
 351                                 LOG_DEBUG_IO("%s scan %d bits; end in %s",
 352                                                 (cmd->cmd.scan->ir_scan) ? "IR" : "DR",
 353                                                 scan_size,
 354                                         tap_state_name(cmd->cmd.scan->end_state));
 355                                 type = jtag_scan_type(cmd->cmd.scan);
 356                                 if (bitbang_scan(cmd->cmd.scan->ir_scan, type, buffer,
 357                                                         scan_size) != ERROR_OK)
 358                                         return ERROR_FAIL;
 359                                 if (jtag_read_buffer(buffer, cmd->cmd.scan) != ERROR_OK)
 360                                         retval = ERROR_JTAG_QUEUE_FAILED;
 361                                 if (buffer)
 362                                         free(buffer);
 363                                 break;
 364                         case JTAG_SLEEP:
 365                                 LOG_DEBUG_IO("sleep %" PRIi32, cmd->cmd.sleep->us);
 366                                 jtag_sleep(cmd->cmd.sleep->us);
 367                                 break;
 368                         case JTAG_TMS:
 369                                 retval = bitbang_execute_tms(cmd);
 370                                 break;
 371                         default:
 372                                 LOG_ERROR("BUG: unknown JTAG command type encountered");
 373                                 exit(-1);
 374                 }
 375                 cmd = cmd->next;
 376         }
 377         if (bitbang_interface->blink) {
 378                 if (bitbang_interface->blink(0) != ERROR_OK)
 379                         return ERROR_FAIL;
 380         }
 381
 382         return retval;
 383 }
 384
 385 static int queued_retval;
 386
 387 static int bitbang_swd_init(void)
 388 {
 389         LOG_DEBUG("bitbang_swd_init");
 390         return ERROR_OK;
 391 }
 392
 393 static void bitbang_swd_exchange(bool rnw, uint8_t buf[], unsigned int offset, unsigned int bit_cnt)
 394 {
 395         LOG_DEBUG("bitbang_swd_exchange");
 396
 397         for (unsigned int i = offset; i < bit_cnt + offset; i++) {
 398                 int bytec = i/8;
 399                 int bcval = 1 << (i % 8);
 400                 int swdio = !rnw && (buf[bytec] & bcval);
 401
 402                 bitbang_interface->swd_write(0, swdio);
 403
 404                 if (rnw && buf) {
 405                         if (bitbang_interface->swdio_read())
 406                                 buf[bytec] |= bcval;
 407                         else
 408                                 buf[bytec] &= ~bcval;
 409                 }
 410
 411                 bitbang_interface->swd_write(1, swdio);
 412         }
 413 }
 414
 415 static int bitbang_swd_switch_seq(enum swd_special_seq seq)
 416 {
 417         LOG_DEBUG("bitbang_swd_switch_seq");
 418
 419         switch (seq) {
 420         case LINE_RESET:
 421                 LOG_DEBUG("SWD line reset");
 422                 bitbang_swd_exchange(false, (uint8_t *)swd_seq_line_reset, 0, swd_seq_line_reset_len);
 423                 break;
 424         case JTAG_TO_SWD:
 425                 LOG_DEBUG("JTAG-to-SWD");
 426                 bitbang_swd_exchange(false, (uint8_t *)swd_seq_jtag_to_swd, 0, swd_seq_jtag_to_swd_len);
 427                 break;
 428         case SWD_TO_JTAG:
 429                 LOG_DEBUG("SWD-to-JTAG");
 430                 bitbang_swd_exchange(false, (uint8_t *)swd_seq_swd_to_jtag, 0, swd_seq_swd_to_jtag_len);
 431                 break;
 432         default:
 433                 LOG_ERROR("Sequence %d not supported", seq);
 434                 return ERROR_FAIL;
 435         }
 436
 437         return ERROR_OK;
 438 }
 439
 440 static void swd_clear_sticky_errors(void)
 441 {
 442         bitbang_swd_write_reg(swd_cmd(false,  false, DP_ABORT),
 443                 STKCMPCLR | STKERRCLR | WDERRCLR | ORUNERRCLR, 0);
 444 }
 445
 446 static void bitbang_swd_read_reg(uint8_t cmd, uint32_t *value, uint32_t ap_delay_clk)
 447 {
 448         LOG_DEBUG("bitbang_swd_read_reg");
 449         assert(cmd & SWD_CMD_RnW);
 450
 451         if (queued_retval != ERROR_OK) {
 452                 LOG_DEBUG("Skip bitbang_swd_read_reg because queued_retval=%d", queued_retval);
 453                 return;
 454         }
 455
 456         for (;;) {
 457                 uint8_t trn_ack_data_parity_trn[DIV_ROUND_UP(4 + 3 + 32 + 1 + 4, 8)];
 458
 459                 cmd |= SWD_CMD_START | (1 << 7);
 460                 bitbang_swd_exchange(false, &cmd, 0, 8);
 461
 462                 bitbang_interface->swdio_drive(false);
 463                 bitbang_swd_exchange(true, trn_ack_data_parity_trn, 0, 1 + 3 + 32 + 1 + 1);
 464                 bitbang_interface->swdio_drive(true);
 465
 466                 int ack = buf_get_u32(trn_ack_data_parity_trn, 1, 3);
 467                 uint32_t data = buf_get_u32(trn_ack_data_parity_trn, 1 + 3, 32);
 468                 int parity = buf_get_u32(trn_ack_data_parity_trn, 1 + 3 + 32, 1);
 469
 470                 LOG_DEBUG("%s %s %s reg %X = %08"PRIx32,
 471                           ack == SWD_ACK_OK ? "OK" : ack == SWD_ACK_WAIT ? "WAIT" : ack == SWD_ACK_FAULT ? "FAULT" : "JUNK",
 472                           cmd & SWD_CMD_APnDP ? "AP" : "DP",
 473                           cmd & SWD_CMD_RnW ? "read" : "write",
 474                           (cmd & SWD_CMD_A32) >> 1,
 475                           data);
 476
 477                 switch (ack) {
 478                  case SWD_ACK_OK:
 479                         if (parity != parity_u32(data)) {
 480                                 LOG_DEBUG("Wrong parity detected");
 481                                 queued_retval = ERROR_FAIL;
 482                                 return;
 483                         }
 484                         if (value)
 485                                 *value = data;
 486                         if (cmd & SWD_CMD_APnDP)
 487                                 bitbang_swd_exchange(true, NULL, 0, ap_delay_clk);
 488                         return;
 489                  case SWD_ACK_WAIT:
 490                         LOG_DEBUG("SWD_ACK_WAIT");
 491                         swd_clear_sticky_errors();
 492                         break;
 493                  case SWD_ACK_FAULT:
 494                         LOG_DEBUG("SWD_ACK_FAULT");
 495                         queued_retval = ack;
 496                         return;
 497                  default:
 498                         LOG_DEBUG("No valid acknowledge: ack=%d", ack);
 499                         queued_retval = ack;
 500                         return;
 501                 }
 502         }
 503 }
 504
 505 static void bitbang_swd_write_reg(uint8_t cmd, uint32_t value, uint32_t ap_delay_clk)
 506 {
 507         LOG_DEBUG("bitbang_swd_write_reg");
 508         assert(!(cmd & SWD_CMD_RnW));
 509
 510         if (queued_retval != ERROR_OK) {
 511                 LOG_DEBUG("Skip bitbang_swd_write_reg because queued_retval=%d", queued_retval);
 512                 return;
 513         }
 514
 515         for (;;) {
 516                 uint8_t trn_ack_data_parity_trn[DIV_ROUND_UP(4 + 3 + 32 + 1 + 4, 8)];
 517                 buf_set_u32(trn_ack_data_parity_trn, 1 + 3 + 1, 32, value);
 518                 buf_set_u32(trn_ack_data_parity_trn, 1 + 3 + 1 + 32, 1, parity_u32(value));
 519
 520                 cmd |= SWD_CMD_START | (1 << 7);
 521                 bitbang_swd_exchange(false, &cmd, 0, 8);
 522
 523                 bitbang_interface->swdio_drive(false);
 524                 bitbang_swd_exchange(true, trn_ack_data_parity_trn, 0, 1 + 3 + 1);
 525                 bitbang_interface->swdio_drive(true);
 526                 bitbang_swd_exchange(false, trn_ack_data_parity_trn, 1 + 3 + 1, 32 + 1);
 527
 528                 int ack = buf_get_u32(trn_ack_data_parity_trn, 1, 3);
 529                 LOG_DEBUG("%s %s %s reg %X = %08"PRIx32,
 530                           ack == SWD_ACK_OK ? "OK" : ack == SWD_ACK_WAIT ? "WAIT" : ack == SWD_ACK_FAULT ? "FAULT" : "JUNK",
 531                           cmd & SWD_CMD_APnDP ? "AP" : "DP",
 532                           cmd & SWD_CMD_RnW ? "read" : "write",
 533                           (cmd & SWD_CMD_A32) >> 1,
 534                           buf_get_u32(trn_ack_data_parity_trn, 1 + 3 + 1, 32));
 535
 536                 switch (ack) {
 537                  case SWD_ACK_OK:
 538                         if (cmd & SWD_CMD_APnDP)
 539                                 bitbang_swd_exchange(true, NULL, 0, ap_delay_clk);
 540                         return;
 541                  case SWD_ACK_WAIT:
 542                         LOG_DEBUG("SWD_ACK_WAIT");
 543                         swd_clear_sticky_errors();
 544                         break;
 545                  case SWD_ACK_FAULT:
 546                         LOG_DEBUG("SWD_ACK_FAULT");
 547                         queued_retval = ack;
 548                         return;
 549                  default:
 550                         LOG_DEBUG("No valid acknowledge: ack=%d", ack);
 551                         queued_retval = ack;
 552                         return;
 553                 }
 554         }
 555 }
 556
 557 static int bitbang_swd_run_queue(void)
 558 {
 559         LOG_DEBUG("bitbang_swd_run_queue");
 560         /* A transaction must be followed by another transaction or at least 8 idle cycles to
 561          * ensure that data is clocked through the AP. */
 562         bitbang_swd_exchange(true, NULL, 0, 8);
 563
 564         int retval = queued_retval;
 565         queued_retval = ERROR_OK;
 566         LOG_DEBUG("SWD queue return value: %02x", retval);
 567         return retval;
 568 }
 569
 570 const struct swd_driver bitbang_swd = {
 571         .init = bitbang_swd_init,
 572         .switch_seq = bitbang_swd_switch_seq,
 573         .read_reg = bitbang_swd_read_reg,
 574         .write_reg = bitbang_swd_write_reg,
 575         .run = bitbang_swd_run_queue,
 576 };