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authorJulius Werner <jwerner@chromium.org>2018-04-24 16:33:58 -0700
committerJulius Werner <jwerner@chromium.org>2018-04-26 01:19:13 +0000
commit1732dcb90ce7e3863c7be28ec9275dd98d4faded (patch)
treee410bb46370608dace1a14631c7cae3e8a6d76af /src/drivers/uart
parenta695a736b4b8a9549e3384b4ec66b148f547d16c (diff)
downloadcoreboot-1732dcb90ce7e3863c7be28ec9275dd98d4faded.tar.xz
drivers/uart: Add helper function to allow bit-banging
In some cases it may be useful to be able to bitbang a UART, such as during early bring-up when a driver for the actual controller isn't available yet. On some platforms we may even want to use this permanently, such as on the SDM845 where the hardware UART controller needs to have firmware loaded and is thus unavailable for most of the bootblock. This patch adds some helper code that makes it easy to implement this on a platform, you just have to pass it a function to control the Tx pin state and it will do the rest. It relies on the mono_time API and is thus bound to microsecond timing granularity, but that seems to be barely good enough for 115200 baud if the bit times are calculated carefully. Change-Id: If7dcecc7b8a95ec15f456efd2ec1f1e0dde239b4 Signed-off-by: Julius Werner <jwerner@chromium.org> Reviewed-on: https://review.coreboot.org/25812 Reviewed-by: Aaron Durbin <adurbin@chromium.org> Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net> Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Diffstat (limited to 'src/drivers/uart')
-rw-r--r--src/drivers/uart/util.c29
1 files changed, 29 insertions, 0 deletions
diff --git a/src/drivers/uart/util.c b/src/drivers/uart/util.c
index c7b9ab21eb..d38f688da2 100644
--- a/src/drivers/uart/util.c
+++ b/src/drivers/uart/util.c
@@ -13,6 +13,8 @@
#include <console/console.h>
#include <console/uart.h>
+#include <types.h>
+#include <timer.h>
/* Calculate divisor. Do not floor but round to nearest integer. */
unsigned int uart_baudrate_divisor(unsigned int baudrate,
@@ -53,3 +55,30 @@ unsigned int uart_platform_refclk(void)
return 115200 * 16;
}
#endif
+
+/* Helper function to allow bitbanging an 8n1 UART. */
+void uart_bitbang_tx_byte(unsigned char data, void (*set_tx)(int line_state))
+{
+ const int baud_rate = get_uart_baudrate();
+ int i;
+ struct stopwatch sw;
+ stopwatch_init(&sw);
+
+ /* Send start bit */
+ set_tx(0);
+ while (stopwatch_duration_usecs(&sw) < MHz / baud_rate)
+ stopwatch_tick(&sw);
+
+ /* 'i' counts the total bits sent at the end of the loop */
+ for (i = 2; i < 10; i++) {
+ set_tx(data & 1);
+ data >>= 1;
+ while (stopwatch_duration_usecs(&sw) < i * MHz / baud_rate)
+ stopwatch_tick(&sw);
+ }
+
+ /* Send stop bit */
+ set_tx(1);
+ while (stopwatch_duration_usecs(&sw) < i * MHz / baud_rate)
+ stopwatch_tick(&sw);
+}