common/platform: fix SWD turnaround

Fix two problems with SWD turnaround:

* When going from output to input, add a missing delay loop, so the
clock low period is consistent. This prevents problems with a missed
clock edge when the clock speed is intentionally slowed to accommodate
lower bandwidth connections to the target.

* Always set the clock low at the end of a turnaround. Otherwise, when
floating the clock after a read transaction, the most recent clock state
was driven high. If there's a pull down on the clock, a subsequent clock
reactivation would drive the clock high, producing an extra rising clock
edge. This sometimes causes (recoverable) protocol error states when
re-attaching after a detach.

Co-Authored-By: Stanislav Sotnikov <stanislav.sotnikov145@gmail.com>

Author: 2 people

src/platforms/common/swdptap.c

@@ -67,17 +67,18 @@ static void swdptap_turnaround(const swdio_status_t dir)
 
 	if (dir == SWDIO_STATUS_FLOAT) {
 		SWDIO_MODE_FLOAT();
-	} else {
+	} else
 		gpio_clear(SWCLK_PORT, SWCLK_PIN);
-		for (volatile uint32_t counter = target_clk_divider + 1; counter > 0; --counter)
-			continue;
-	}
+
+	for (volatile uint32_t counter = target_clk_divider + 1; counter > 0; --counter)
+		continue;
 
 	gpio_set(SWCLK_PORT, SWCLK_PIN);
 	for (volatile uint32_t counter = target_clk_divider + 1; counter > 0; --counter)
 		continue;
 
 	if (dir == SWDIO_STATUS_DRIVE) {
+		gpio_clear(SWCLK_PORT, SWCLK_PIN);
 		SWDIO_MODE_DRIVE();
 	}
 }