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author | Tomas Vanek <vanekt@fbl.cz> | 2022-10-14 09:19:29 +0200 |
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committer | Antonio Borneo <borneo.antonio@gmail.com> | 2023-01-28 15:53:21 +0000 |
commit | 148bc7e2151740527b4ca67d0a7c7c9f01725981 (patch) | |
tree | 5a5ddbdc0ea61b2e14f9bb35436673673969c078 /src | |
parent | 20285b91008106c9fa966cea3269c6f6a81e539a (diff) | |
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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>
Diffstat (limited to 'src')
-rw-r--r-- | src/jtag/drivers/bitbang.c | 14 |
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); |