cpu/stm32_common: added SPI div calc tool

cpu/stm32_common/dist/spi_divtable/Makefile

@@ -0,0 +1,12 @@
+NAME        = spi_divtable
+CC          = gcc
+CFLAGS      = -std=c99 -Wall
+SRC         = $(wildcard *.c)
+
+.PHONY: all clean
+
+all:
+	$(CC) $(CFLAGS) -o $(NAME) $(SRC)
+
+clean:
+	rm -f $(NAME)

cpu/stm32_common/dist/spi_divtable/spi_divtable.c

@@ -0,0 +1,105 @@
+/*
+ * Copyright (C) 2016 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.
+ */
+
+/**
+ * @brief       Compute SPI clock scaler values for STM32x CPUs
+ *
+ * This helper tool calculates the needed SPI scaler values for the APBx buses.
+ * It outputs the values for the possible SPI clock speeds based on the clock
+ * speeds of the APB1 and the APB2 bus.
+ *
+ * @author      Hauke Petersen <hauke.petersen@fu-berlin.de>
+ *
+ * @}
+ */
+
+#include <stdint.h>
+#include <stdlib.h>
+#include <stdio.h>
+
+
+static int targets[] = { 100000, 400000, 1000000, 5000000, 10000000 };
+
+/**
+ * @brief   Find the best fitting divisor value base on target and APB clock
+ *
+ * The SPI bus clock speed is calculated as follows:
+ *
+ * clk_bus = clk_apb / 2 ^ (x + 1)
+ *
+ * In this function we will try to find the x-value, that brings clk_bus as
+ * close as possible to the value given in @p target.
+ *
+ * @param[in] bus       clock speed of the given APB bus
+ * @param[in] target    targeted SPI clock speed
+ *
+ * @return  the closes divisor value to be used in the clock config register
+ */
+static int find_best(int bus, int target)
+{
+    int br = -1;
+    int tmp = bus;
+
+    do {
+        // printf("b: x: %i - tmp: %i\n", br, tmp);
+        ++br;
+        tmp /= 2;
+        if (tmp == target) {
+            return br;
+        }
+        // printf("a: x: %i - tmp: %i\n", br, tmp);
+        // printf("  (tmp - target):%i\n", (tmp - target));
+    } while ((tmp - target > 0) && (br < 7));
+
+    int old = tmp * 2;
+    // printf("(target - tmp):%i, (old - target): %i\n", (target - tmp), (old - target));
+
+    if ((target - tmp) > (old - target)) {
+        return (br - 1);
+    }
+    return br;
+}
+
+static int real_clk(int bus, int br)
+{
+    return bus / (2 << br);
+}
+
+int main(int argc, char **argv)
+{
+    int tnum = sizeof(targets) / sizeof(targets[0]);
+    int apb[2];
+
+    if (argc != 3) {
+        printf("usage: %s <APB1 clk> <APB2 clk> (in Hz)\n", argv[0]);
+        return 1;
+    }
+
+    apb[0] = atoi(argv[1]); /* APB1 */
+    apb[1] = atoi(argv[2]); /* APB2 */
+
+    if ((apb[0] <= 0) || (apb[1] <= 0)) {
+        puts("error: invalid APB clock speed values");
+        return 1;
+    }
+
+    printf("static const uint8_t spi_divtable[2][%i] = {\n", tnum);
+
+    for (int bus = 0; bus < (sizeof(apb) / sizeof(apb[0])); bus ++) {
+        printf("    {       /* for APB%i @ %iHz */\n", (bus + 1), apb[bus]);
+        for (int t = 0; t < tnum; t++) {
+            int br = find_best(apb[bus], targets[t]);
+            printf("        %i%c  /* -> %iHz */\n",
+                   br, ((t < (tnum - 1)) ? ',' : ' '), real_clk(apb[bus], br));
+        }
+        printf("    }%c\n", ((bus == 0) ? ',' : ' '));
+    }
+    puts("};");
+
+    return 0;
+}