samd21: Add support for basic i2c support (write only)

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/samd21/Kconfig

@@ -6,6 +6,7 @@ config SAMD_SELECT
     bool
     default y
     select HAVE_GPIO
+    select HAVE_GPIO_I2C
     select HAVE_GPIO_BITBANGING
 
 config BOARD_DIRECTORY

src/samd21/Makefile

@@ -20,6 +20,7 @@ src-y += generic/armcm_irq.c generic/timer_irq.c
 src-y += ../lib/samd21/samd21a/gcc/gcc/startup_samd21.c
 src-$(CONFIG_USBSERIAL) += samd21/usbserial.c generic/usb_cdc.c
 src-$(CONFIG_SERIAL) += samd21/serial.c generic/serial_irq.c
+src-$(CONFIG_HAVE_GPIO_I2C) += samd21/i2c.c
 
 # Support bootloader offset address
 target-y := $(OUT)samd21.ld $(target-y)

src/samd21/gpio.h

@@ -21,4 +21,13 @@ struct gpio_in gpio_in_setup(uint8_t pin, int8_t pull_up);
 void gpio_in_reset(struct gpio_in g, int8_t pull_up);
 uint8_t gpio_in_read(struct gpio_in g);
 
+struct i2c_config {
+    uint8_t addr;
+};
+
+struct i2c_config i2c_setup(uint32_t bus, uint32_t rate, uint8_t addr);
+void i2c_write(struct i2c_config config, uint8_t write_len, uint8_t *write);
+void i2c_read(struct i2c_config config, uint8_t reg_len, uint8_t *reg
+              , uint8_t read_len, uint8_t *read);
+
 #endif // gpio.h

src/samd21/i2c.c

@@ -0,0 +1,114 @@
+// i2c support on samd21
+//
+// Copyright (C) 2018  Kevin O'Connor <kevin@koconnor.net>
+//
+// This file may be distributed under the terms of the GNU GPLv3 license.
+
+#include "autoconf.h" // CONFIG_CLOCK_FREQ
+#include "internal.h" // enable_pclock
+#include "command.h" // shutdown
+#include "gpio.h" // i2c_setup
+#include "samd21.h" // SERCOM3
+#include "sched.h" // sched_shutdown
+
+#define TIME_RISE 125ULL // 125 nanoseconds
+#define I2C_FREQ 100000
+
+static void
+i2c_init(void)
+{
+    static int have_run_init;
+    if (have_run_init)
+        return;
+    have_run_init = 1;
+
+    // Setup clock
+    enable_pclock(SERCOM3_GCLK_ID_CORE, PM_APBCMASK_SERCOM3);
+
+    // Configure SDA, SCL pins
+    gpio_peripheral(GPIO('A', 22), 'C', 0);
+    gpio_peripheral(GPIO('A', 23), 'C', 0);
+
+    // Configure i2c
+    SercomI2cm *si = &SERCOM3->I2CM;
+    si->CTRLA.reg = 0;
+    uint32_t areg = (SERCOM_I2CM_CTRLA_LOWTOUTEN
+                     | SERCOM_I2CM_CTRLA_INACTOUT(3)
+                     | SERCOM_I2CM_STATUS_SEXTTOUT
+                     | SERCOM_I2CM_STATUS_MEXTTOUT
+                     | SERCOM_I2CM_CTRLA_MODE_I2C_MASTER);
+    si->CTRLA.reg = areg;
+    uint32_t baud = (CONFIG_CLOCK_FREQ / I2C_FREQ
+                     - 10 - CONFIG_CLOCK_FREQ*TIME_RISE/1000000000) / 2;
+    si->BAUD.reg = baud;
+    si->CTRLA.reg = areg | SERCOM_I2CM_CTRLA_ENABLE;
+    while (si->SYNCBUSY.reg & SERCOM_I2CM_SYNCBUSY_ENABLE)
+        ;
+
+    // Go into idle mode
+    si->STATUS.reg = SERCOM_I2CM_STATUS_BUSSTATE(1);
+    while (si->SYNCBUSY.reg & SERCOM_I2CM_SYNCBUSY_SYSOP)
+        ;
+}
+
+struct i2c_config
+i2c_setup(uint32_t bus, uint32_t rate, uint8_t addr)
+{
+    if (bus)
+        shutdown("Unsupported i2c bus");
+    i2c_init();
+    return (struct i2c_config){ .addr=addr };
+}
+
+static void
+i2c_wait(SercomI2cm *si)
+{
+    for (;;) {
+        uint32_t intflag = si->INTFLAG.reg;
+        if (!(intflag & SERCOM_I2CM_INTFLAG_MB)) {
+            if (si->STATUS.reg & SERCOM_I2CM_STATUS_BUSERR)
+                shutdown("i2c buserror");
+            continue;
+        }
+        if (intflag & SERCOM_I2CM_INTFLAG_ERROR)
+            shutdown("i2c error");
+        break;
+    }
+}
+
+static void
+i2c_start(SercomI2cm *si, uint8_t addr)
+{
+    si->ADDR.reg = addr;
+    i2c_wait(si);
+}
+
+static void
+i2c_send_byte(SercomI2cm *si, uint8_t b)
+{
+    si->DATA.reg = b;
+    i2c_wait(si);
+}
+
+static void
+i2c_stop(SercomI2cm *si)
+{
+    si->CTRLB.reg = SERCOM_I2CM_CTRLB_CMD(3);
+}
+
+void
+i2c_write(struct i2c_config config, uint8_t write_len, uint8_t *write)
+{
+    SercomI2cm *si = &SERCOM3->I2CM;
+    i2c_start(si, config.addr);
+    while (write_len--)
+        i2c_send_byte(si, *write++);
+    i2c_stop(si);
+}
+
+void
+i2c_read(struct i2c_config config, uint8_t reg_len, uint8_t *reg
+         , uint8_t read_len, uint8_t *read)
+{
+    shutdown("i2c_read not supported on samd21");
+}