cpu/sam3: add periph_rtt driver implementation

cpu/sam3/include/periph_cpu.h

@@ -70,6 +70,11 @@ typedef uint32_t gpio_t;
  */
 #define TIMER_CHANNELS      (3)
 
+/**
+ * @brief   The RTT width is fixed to 32-bit
+ */
+#define RTT_MAX_VALUE       (0xffffffff)
+
 /**
  * @brief   Generate GPIO mode bitfields
  *

cpu/sam3/periph/rtt.c

@@ -0,0 +1,107 @@
+/*
+ * Copyright (C) 2017,2020 Freie Universität Berlin
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @ingroup     cpu_sam3
+ * @ingroup     drivers_periph_rtt
+ *
+ * @note        The hardware RTT unit does neither support overflow interrupts
+ *              nor setting the counter value. For this, this RTT driver does
+ *              not implement those functions.
+ * @{
+ *
+ * @file
+ * @brief       Low-level RTT driver implementation
+ *
+ * @author      Hauke Petersen <hauke.petersen@fu-berlin.de>
+ *
+ * @}
+ */
+
+#include "cpu.h"
+#include "periph/rtt.h"
+
+#define ENABLE_DEBUG        (0)
+#include "debug.h"
+
+static struct {
+    rtt_cb_t cb;
+    void *arg;
+} isr_ctx;
+
+void rtt_init(void)
+{
+    /* enable RTT module */
+    rtt_poweron();
+
+    /* configure and apply the pre-scaler */
+    RTT->RTT_MR = RTT_MR_RTPRES(CHIP_FREQ_XTAL_32K / RTT_FREQUENCY);
+    RTT->RTT_MR |= RTT_MR_RTTRST;
+
+    /* resetting the timer takes two slow clock cycles, so we wait for this to
+     * complete */
+    while (RTT->RTT_VR != 0) {}
+
+    /* configure NVIC line */
+    NVIC_EnableIRQ(RTT_IRQn);
+}
+
+uint32_t rtt_get_counter(void)
+{
+    return RTT->RTT_VR;
+}
+
+void rtt_set_alarm(uint32_t alarm, rtt_cb_t cb, void *arg)
+{
+    /* cancel any existing alarm */
+    RTT->RTT_MR &= ~(RTT_MR_ALMIEN);
+
+    /* set new alarm */
+    isr_ctx.cb  = cb;
+    isr_ctx.arg = arg;
+    /* the alarm value is RTT_AR + 1, so we need to subtract 1 from the target
+     * value here */
+    RTT->RTT_AR = (alarm - 1);
+
+    /* (re-)enable the alarm */
+    RTT->RTT_MR |= RTT_MR_ALMIEN;
+}
+
+uint32_t rtt_get_alarm(void)
+{
+    if (RTT->RTT_MR & RTT_MR_ALMIEN) {
+        return RTT->RTT_AR;
+    }
+    return 0;
+}
+
+void rtt_clear_alarm(void)
+{
+    RTT->RTT_MR &= ~(RTT_MR_ALMIEN);
+}
+
+void rtt_poweron(void)
+{
+    PMC->PMC_PCER0 |= (1 << ID_RTT);
+}
+
+void rtt_poweroff(void)
+{
+    PMC->PMC_PCER0 &= ~(1 << ID_RTT);
+}
+
+void isr_rtt(void)
+{
+    uint32_t state = RTT->RTT_SR;       /* this clears all pending flags */
+    if (state & RTT_SR_ALMS) {
+        RTT->RTT_MR &= ~(RTT_MR_ALMIEN);
+        isr_ctx.cb(isr_ctx.arg);
+    }
+
+    cortexm_isr_end();
+}