stlink: remove reset pulse when entering in JTAG

Until version J14 the behaviour of ST-Link was to send a reset
pulse to the target when the debug connection is started in JTAG
mode. No reset pulse is sent, instead, in SWD mode.
Version J15 introduces a new parameter to avoid the reset pulse in
JTAG mode, aligning the behaviour with SWD.

This reset from the ST-Link, if propagated to the target, prevents
attaching a running target.

Actually this reset pulse is very short (few microsecond) and can
be easily filtered out by an on-board capacitor, usually present
on the reset wire (mainly to filter the bounces of the reset
button). Moreover, most of the use cases for ST-Link are with SWD
(not with JTAG) and this has probably further masked this JTAG
specific behaviour.

OpenOCD can tolerate it but requires the flag "connect_assert_srst"
to the command "reset_config", but the flag is not present in any
configurations in folder tcl. This enforces the guess it was not
noticed due to on-board capacitors or missing connection of reset
pin or ST-Link only used in SWD; so it's safe applying this patch.

Change the default behaviour to avoid reset in JTAG at connection.
There is no need to manage the ST-Link version here, since every
parameter that is not recognized by older ST-Link is treated as
"connect in JTAG with reset pulse", keeping backward compatibility.

Change-Id: Idc97a1457279e3970fd0839cadbff22d9b0302d4
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4713
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

diff --git a/src/jtag/drivers/stlink_usb.c b/src/jtag/drivers/stlink_usb.c
index 5c5fde952177b4ac48ff83c1e346297a3ec44bc8..f05ffadf9e1896257426df3252ba2c7a318286d5 100644 (file)
--- a/src/jtag/drivers/stlink_usb.c
+++ b/src/jtag/drivers/stlink_usb.c
@@ -205,7 +205,6 @@ struct stlink_usb_handle_s {
 #define STLINK_SWIM_READMEM                0x0b
 #define STLINK_SWIM_READBUF                0x0c
 
-#define STLINK_DEBUG_ENTER_JTAG            0x00
 #define STLINK_DEBUG_GETSTATUS             0x01
 #define STLINK_DEBUG_FORCEDEBUG            0x02
 #define STLINK_DEBUG_APIV1_RESETSYS        0x03
@@ -223,8 +222,9 @@ struct stlink_usb_handle_s {
 #define STLINK_DEBUG_APIV1_WRITEDEBUGREG   0x0f
 #define STLINK_DEBUG_APIV1_SETWATCHPOINT   0x10
 
-#define STLINK_DEBUG_ENTER_JTAG            0x00
-#define STLINK_DEBUG_ENTER_SWD             0xa3
+#define STLINK_DEBUG_ENTER_JTAG_RESET      0x00
+#define STLINK_DEBUG_ENTER_SWD_NO_RESET    0xa3
+#define STLINK_DEBUG_ENTER_JTAG_NO_RESET   0xa4
 
 #define STLINK_DEBUG_APIV1_ENTER           0x20
 #define STLINK_DEBUG_EXIT                  0x21
@@ -830,7 +830,7 @@ static int stlink_usb_mode_enter(void *handle, enum stlink_mode type)
                                h->cmdbuf[h->cmdidx++] = STLINK_DEBUG_APIV1_ENTER;
                        else
                                h->cmdbuf[h->cmdidx++] = STLINK_DEBUG_APIV2_ENTER;
-                       h->cmdbuf[h->cmdidx++] = STLINK_DEBUG_ENTER_JTAG;
+                       h->cmdbuf[h->cmdidx++] = STLINK_DEBUG_ENTER_JTAG_NO_RESET;
                        break;
                case STLINK_MODE_DEBUG_SWD:
                        h->cmdbuf[h->cmdidx++] = STLINK_DEBUG_COMMAND;
@@ -838,7 +838,7 @@ static int stlink_usb_mode_enter(void *handle, enum stlink_mode type)
                                h->cmdbuf[h->cmdidx++] = STLINK_DEBUG_APIV1_ENTER;
                        else
                                h->cmdbuf[h->cmdidx++] = STLINK_DEBUG_APIV2_ENTER;
-                       h->cmdbuf[h->cmdidx++] = STLINK_DEBUG_ENTER_SWD;
+                       h->cmdbuf[h->cmdidx++] = STLINK_DEBUG_ENTER_SWD_NO_RESET;
                        break;
                case STLINK_MODE_DEBUG_SWIM:
                        h->cmdbuf[h->cmdidx++] = STLINK_SWIM_COMMAND;