STM: optimize us_ticker

Provide new optimizations for us_ticker and wait_us.

Author: kjbracey

targets/TARGET_STM/device.h

@@ -37,4 +37,6 @@
 
 #include "objects.h"
 
+#include "us_ticker_defines.h"
+
 #endif

targets/TARGET_STM/us_ticker.c

@@ -20,6 +20,7 @@
 #include "us_ticker_api.h"
 #include "PeripheralNames.h"
 #include "us_ticker_data.h"
+#include "us_ticker_defines.h"
 
 TIM_HandleTypeDef TimMasterHandle;
 
@@ -212,9 +213,10 @@ void us_ticker_init(void)
     HAL_TIM_OC_Start(&TimMasterHandle, TIM_CHANNEL_1);
 }
 
-uint32_t us_ticker_read()
+uint32_t (us_ticker_read)()
 {
-    return TIM_MST->CNT;
+    /* Invoke the macro */
+    return us_ticker_read();
 }
 
 void us_ticker_set_interrupt(timestamp_t timestamp)

targets/TARGET_STM/us_ticker_defines.h

@@ -0,0 +1,37 @@
+/* mbed Microcontroller Library
+ * Copyright (c) 2006-2019 ARM Limited
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#ifndef MBED_US_TICKER_DEFINES_H
+#define MBED_US_TICKER_DEFINES_H_
+
+#include "us_ticker_data.h"
+
+/* All devices have 1MHz us_ticker */
+#define US_TICKER_PERIOD_NUM    1
+#define US_TICKER_PERIOD_DEN    1
+
+#if TIM_MST_BIT_WIDTH == 16
+#define US_TICKER_MASK          0xFFFF
+#elif TIM_MST_BIT_WIDTH == 32
+#define US_TICKER_MASK          0xFFFFFFFF
+#else
+#error "Bad TIM_MST_BIT_WIDTH"
+#endif
+
+/* Macro-optimised form of us_ticker_read */
+#define us_ticker_read() (TIM_MST->CNT)
+
+#endif /* MBED_US_TICKER_DEFINES_H_ */