Huge refactoring for C++ and low-power mode

The display currently shows the time, with hours and minutes, and is
capable of receiving input with buttons (though does nothing). It
sleeps during intervals where nothing is happening. The display task
runs once per second, and RTC alarm A is used for periodic alarms to
update the system time.

LptimPwm.cpp

@@ -0,0 +1,111 @@
+/*
+ * Copyright (C) 2019  Max Regan
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#include "LptimPwm.h"
+#include "macros.h"
+
+namespace BSP {
+
+using Common::ReturnCode;
+
+LptimPwm::LptimPwm(LPTIM_TypeDef *lptim)
+    : m_lptim(lptim)
+{}
+
+
+void LptimPwm::init_lptim()
+{
+    /* Enable LPTIM in APB1 */
+    SET(RCC->APB1ENR,
+        RCC_APB1ENR_LPTIM1EN);
+
+    // Enable low-speed internal
+    RCC->CSR |= RCC_CSR_LSION;
+    while (!(RCC->CSR & RCC_CSR_LSIRDY)) {};
+
+    /*!< Set the LSE clock to be the source of the LPTIM */
+    SET_TO(RCC->CCIPR,
+           RCC_CCIPR_LPTIM1SEL,
+           RCC_CCIPR_LPTIM1SEL_0);
+
+    /** Write CR CFGR and IER while LPTIM is disabled (LPTIM_CR_ENABLE not yet set) */
+    /*!< Disable Interrupts (not needed, this is the default */
+    LPTIM1->IER = 0;
+
+    /*!< Reset
+     * ENC (Disable encoder mode)
+     * TIMOUT (disable timeout mode)
+     * TRIGEN (Trigger count start with software only)
+     * PRELOAD (Update ARR and CMP registers immediately after write)
+     * CKSEL (LPTIM is not using an input clock)
+     * COUNTMODE (LPTIM counter updated on every clock pulse)
+     * TRGFLT (Do not debounce triggers)
+     */
+    CLR(LPTIM1->CFGR,
+          LPTIM_CFGR_ENC | LPTIM_CFGR_TIMOUT | LPTIM_CFGR_TRIGEN |
+          LPTIM_CFGR_TRIGSEL | LPTIM_CFGR_PRELOAD | LPTIM_CFGR_CKSEL |
+          LPTIM_CFGR_COUNTMODE);
+
+    /*!< Set
+     * PRESC (Set prescaler to 128. Using 32kHz LSE as input, this should
+     *    correspond to 250Hz counting.
+     */
+    CLR(LPTIM1->CFGR, LPTIM_CFGR_PRESC);
+
+    SET(LPTIM1->CR, LPTIM_CR_ENABLE);
+
+    /*!< Do not modify ARR and CMP until after ENABLE bit is set */
+    /*!< Produce a 60Hz, signal with minimal "high" time. The display
+       only needs 2us of "high" time on EXTCOMM, and it draws a fair
+       amount of power. */
+    LPTIM1->ARR = 0x4FF;
+    LPTIM1->CMP = 0x4FE;
+    while(!(LPTIM1->ISR & LPTIM_ISR_ARROK)) {}
+    while(!(LPTIM1->ISR & LPTIM_ISR_CMPOK)) {}
+
+    /*!< Enable and start the timer */
+    SET(LPTIM1->CR, LPTIM_CR_CNTSTRT);
+}
+
+
+ReturnCode LptimPwm::init()
+{
+    init_lptim();
+
+    /* Enable GPIO port A */
+    SET(RCC->IOPENR, RCC_IOPENR_IOPAEN);
+
+    /* Assign LPTIM1_OUT to PA7 */
+    SET_TO(GPIOA->AFR[0],
+           GPIO_AFRL_AFRL7,
+           1u << GPIO_AFRL_AFRL7_Pos);
+
+    SET_TO(GPIOA->MODER,
+           GPIO_MODER_MODE7,
+           2u << GPIO_MODER_MODE7_Pos);
+
+    CLR(GPIOA->OTYPER, GPIO_OTYPER_OT_7);
+    CLR(GPIOA->PUPDR, GPIO_PUPDR_PUPD7);
+
+    return ReturnCode::OK;
+}
+
+}