diff --git a/src/jtag/jtag.c b/src/jtag/jtag.c
index fc89d2489..7d0dcab9d 100644
--- a/src/jtag/jtag.c
+++ b/src/jtag/jtag.c
@@ -1411,7 +1411,7 @@ int jtag_register_commands(struct command_context_s *cmd_ctx)
 	register_command(cmd_ctx, NULL, "jtag_speed", handle_jtag_speed_command,
 		COMMAND_ANY, "set jtag speed (if supported) <reset speed> [<post reset speed, default value is reset speed>]");
 	register_command(cmd_ctx, NULL, "jtag_khz", handle_jtag_khz_command,
-		COMMAND_ANY, "same as jtag_speed, except it takes khz as arguments");
+		COMMAND_ANY, "same as jtag_speed, except it takes maximum khz as arguments. 0 KHz = RTCK.");
 	register_command(cmd_ctx, NULL, "jtag_device", handle_jtag_device_command,
 		COMMAND_CONFIG, "jtag_device <ir_length> <ir_expected> <ir_mask>");
 	register_command(cmd_ctx, NULL, "reset_config", handle_reset_config_command,
diff --git a/src/jtag/jtag.h b/src/jtag/jtag.h
index 16eb84dd0..fca7c3500 100644
--- a/src/jtag/jtag.h
+++ b/src/jtag/jtag.h
@@ -195,7 +195,8 @@ typedef struct jtag_interface_s
 	int (*register_commands)(struct command_context_s *cmd_ctx);
 	int (*init)(void);
 	int (*quit)(void);
-	/* returns JTAG speed for KHz. */
+	/* returns JTAG maxium speed for KHz. 0=RTCK. The function returns
+	a failure if it can't support the KHz/RTCK. */
 	int (*khz)(int khz, int *jtag_speed);
 	
 } jtag_interface_t;