jtag/drivers/bitbang: avoid mostly harmless glitch on SWDIO

bitbang_swd_exchange(rnw=true,...) calls bitbang_interface->swd_write()
with swdio clamped to 0.
bitbang_swd_write_reg() reads 1 turnaround bit, 3 ack bits
and 1 turnaround by one call to bitbang_swd_exchange()
and then switches SWDIO to output.
AFAIK all bitbang interfaces switch SWDIO GPIO direction immediately
in bitbang_interface->swdio_drive().
The GPIO now drives SWDIO line to the value stored in the output register
which is always zero from previous bitbang_swd_exchange(rnw=true,...).
In case the following data bit (bit 0) is 1 we can observe a glitch
on SWDIO:
                                         _____ out 1 ____
HiZ/pull-up ----\                       /
                 \                     /
                  \______ out 0 ______/
          swdio_drive(true)   swd_write(0,1)

The glitch fortunately takes place far enough from SWCLK rising edge
where SWDIO is sampled by the target, so I believe it is harmless
except some corner cases where the reflected wave is delayed on long
line.

Anyway keeping electrical signals glitch free is a good practice.
To keep performance penalty minimal, pre-write the first data
bit to SWDIO GPIO output buffer while clocking the turnaround bit.
Following swdio_drive(true) outputs the pre-written value
and the same value is rewritten by the next swd_write()
instead of glitching SWDIO.

Change-Id: I72ea9c0b2fae57e8ff5aa616859182c67abc924f
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/7260
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

1 files changed, 13 insertions, 1 deletions

diff --git a/src/jtag/drivers/bitbang.c b/src/jtag/drivers/bitbang.c
index 2ab0a2a..665dbf3 100644
--- a/src/jtag/drivers/bitbang.c
+++ b/src/jtag/drivers/bitbang.c
@@ -525,7 +525,19 @@ static void bitbang_swd_write_reg(uint8_t cmd, uint32_t value, uint32_t ap_delay
 		bitbang_swd_exchange(false, &cmd, 0, 8);
 
 		bitbang_interface->swdio_drive(false);
-		bitbang_swd_exchange(true, trn_ack_data_parity_trn, 0, 1 + 3 + 1);
+		bitbang_swd_exchange(true, trn_ack_data_parity_trn, 0, 1 + 3);
+
+		/* Avoid a glitch on SWDIO when changing the direction to output.
+		 * To keep performance penalty minimal, pre-write the first data
+		 * bit to SWDIO GPIO output buffer while clocking the turnaround bit.
+		 * Following swdio_drive(true) outputs the pre-written value
+		 * and the same value is rewritten by the next swd_write()
+		 * instead of glitching SWDIO
+		 * HiZ/pull-up --------------> 0 -------------> 1
+		 *           swdio_drive(true)   swd_write(0,1)
+		 * in case of data bit 0 = 1
+		 */
+		bitbang_swd_exchange(false, trn_ack_data_parity_trn, 1 + 3 + 1, 1);
 		bitbang_interface->swdio_drive(true);
 		bitbang_swd_exchange(false, trn_ack_data_parity_trn, 1 + 3 + 1, 32 + 1);