jtag ftdi: sample TDO on falling edge of TCK

Due to signal propagation delays, sampling TDO on rising TCK can become
quite peculiar at fast TCK rates. However, FTDI chips offer a possiblity
to sample TDO on falling edge. With this change, stable operation can be
achieved at 30MHz clock even over 10cm ribbon cable.

Change-Id: Icaf240535dae15512e3c60a944e22a5fbc1b0b06
Signed-off-by: Matthias Welwarsky <matthias@welwarsky.de>
Reviewed-on: http://openocd.zylin.com/3180
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>

doc/openocd.texi

@@ -2610,8 +2610,22 @@ Set a previously defined signal to the specified level.
 @end itemize
 @end deffn
 
+@deffn {Command} {ftdi_tdo_sample_edge} @option{rising}|@option{falling}
+Configure TCK edge at which the adapter samples the value of the TDO signal
+
+Due to signal propagation delays, sampling TDO on rising TCK can become quite
+peculiar at high JTAG clock speeds. However, FTDI chips offer a possiblity to sample
+TDO on falling edge of TCK. With some board/adapter configurations, this may increase
+stability at higher JTAG clocks.
+@itemize @minus
+@item @option{rising}, sample TDO on rising edge of TCK - this is the default
+@item @option{falling}, sample TDO on falling edge of TCK
+@end itemize
+@end deffn
+
 For example adapter definitions, see the configuration files shipped in the
 @file{interface/ftdi} directory.
+
 @end deffn
 
 @deffn {Interface Driver} {remote_bitbang}

src/jtag/drivers/ftdi.c

@@ -86,11 +86,13 @@
 #include "mpsse.h"
 
 #define JTAG_MODE (LSB_FIRST | POS_EDGE_IN | NEG_EDGE_OUT)
+#define JTAG_MODE_ALT (LSB_FIRST | NEG_EDGE_IN | NEG_EDGE_OUT)
 #define SWD_MODE (LSB_FIRST | POS_EDGE_IN | NEG_EDGE_OUT)
 
 static char *ftdi_device_desc;
 static char *ftdi_serial;
 static uint8_t ftdi_channel;
+static uint8_t ftdi_jtag_mode = JTAG_MODE;
 
 static bool swd_mode;
 
@@ -244,7 +246,7 @@ static void move_to_state(tap_state_t goal_state)
 		0,
 		tms_count,
 		false,
-		JTAG_MODE);
+		ftdi_jtag_mode);
 }
 
 static int ftdi_speed(int speed)
@@ -257,6 +259,9 @@ static int ftdi_speed(int speed)
 		return retval;
 	}
 
+	if (!swd_mode && speed >= 10000000 && ftdi_jtag_mode != JTAG_MODE_ALT)
+		LOG_INFO("ftdi: if you experience problems at higher adapter clocks, try "
+			 "the command \"ftdi_tdo_sample_edge falling\"");
 	return ERROR_OK;
 }
 
@@ -304,7 +309,7 @@ static void ftdi_execute_runtest(struct jtag_command *cmd)
 	while (i > 0) {
 		/* there are no state transitions in this code, so omit state tracking */
 		unsigned this_len = i > 7 ? 7 : i;
-		mpsse_clock_tms_cs_out(mpsse_ctx, &zero, 0, this_len, false, JTAG_MODE);
+		mpsse_clock_tms_cs_out(mpsse_ctx, &zero, 0, this_len, false, ftdi_jtag_mode);
 		i -= this_len;
 	}
 
@@ -344,7 +349,7 @@ static void ftdi_execute_tms(struct jtag_command *cmd)
 		0,
 		cmd->cmd.tms->num_bits,
 		false,
-		JTAG_MODE);
+		ftdi_jtag_mode);
 }
 
 static void ftdi_execute_pathmove(struct jtag_command *cmd)
@@ -391,7 +396,7 @@ static void ftdi_execute_pathmove(struct jtag_command *cmd)
 					0,
 					bit_count,
 					false,
-					JTAG_MODE);
+					ftdi_jtag_mode);
 			bit_count = 0;
 		}
 	}
@@ -446,7 +451,7 @@ static void ftdi_execute_scan(struct jtag_command *cmd)
 				field->in_value,
 				0,
 				field->num_bits - 1,
-				JTAG_MODE);
+				ftdi_jtag_mode);
 			uint8_t last_bit = 0;
 			if (field->out_value)
 				bit_copy(&last_bit, 0, field->out_value, field->num_bits - 1, 1);
@@ -458,14 +463,14 @@ static void ftdi_execute_scan(struct jtag_command *cmd)
 					field->num_bits - 1,
 					1,
 					last_bit,
-					JTAG_MODE);
+					ftdi_jtag_mode);
 			tap_set_state(tap_state_transition(tap_get_state(), 1));
 			mpsse_clock_tms_cs_out(mpsse_ctx,
 					&tms_bits,
 					1,
 					1,
 					last_bit,
-					JTAG_MODE);
+					ftdi_jtag_mode);
 			tap_set_state(tap_state_transition(tap_get_state(), 0));
 		} else
 			mpsse_clock_data(mpsse_ctx,
@@ -474,7 +479,7 @@ static void ftdi_execute_scan(struct jtag_command *cmd)
 				field->in_value,
 				0,
 				field->num_bits,
-				JTAG_MODE);
+				ftdi_jtag_mode);
 	}
 
 	if (tap_get_state() != tap_get_end_state())
@@ -553,7 +558,7 @@ static void ftdi_execute_stableclocks(struct jtag_command *cmd)
 	while (num_cycles > 0) {
 		/* there are no state transitions in this code, so omit state tracking */
 		unsigned this_len = num_cycles > 7 ? 7 : num_cycles;
-		mpsse_clock_tms_cs_out(mpsse_ctx, &tms, 0, this_len, false, JTAG_MODE);
+		mpsse_clock_tms_cs_out(mpsse_ctx, &tms, 0, this_len, false, ftdi_jtag_mode);
 		num_cycles -= this_len;
 	}
 
@@ -832,6 +837,29 @@ COMMAND_HANDLER(ftdi_handle_vid_pid_command)
 	return ERROR_OK;
 }
 
+COMMAND_HANDLER(ftdi_handle_tdo_sample_edge_command)
+{
+	Jim_Nvp *n;
+	static const Jim_Nvp nvp_ftdi_jtag_modes[] = {
+		{ .name = "rising", .value = JTAG_MODE },
+		{ .name = "falling", .value = JTAG_MODE_ALT },
+		{ .name = NULL, .value = -1 },
+	};
+
+	if (CMD_ARGC > 0) {
+		n = Jim_Nvp_name2value_simple(nvp_ftdi_jtag_modes, CMD_ARGV[0]);
+		if (n->name == NULL)
+			return ERROR_COMMAND_SYNTAX_ERROR;
+		ftdi_jtag_mode = n->value;
+
+	}
+
+	n = Jim_Nvp_value2name_simple(nvp_ftdi_jtag_modes, ftdi_jtag_mode);
+	command_print(CMD_CTX, "ftdi samples TDO on %s edge of TCK", n->name);
+
+	return ERROR_OK;
+}
+
 static const struct command_registration ftdi_command_handlers[] = {
 	{
 		.name = "ftdi_device_desc",
@@ -884,6 +912,15 @@ static const struct command_registration ftdi_command_handlers[] = {
 		.help = "the vendor ID and product ID of the FTDI device",
 		.usage = "(vid pid)* ",
 	},
+	{
+		.name = "ftdi_tdo_sample_edge",
+		.handler = &ftdi_handle_tdo_sample_edge_command,
+		.mode = COMMAND_ANY,
+		.help = "set which TCK clock edge is used for sampling TDO "
+			"- default is rising-edge (Setting to falling-edge may "
+			"allow signalling speed increase)",
+		.usage = "(rising|falling)",
+	},
 	COMMAND_REGISTRATION_DONE
 };