minimal: Add enough code to run minimal build on STM32F4xx hardware.

Minimal support code for a Cortex-M CPU is added, along with set-up
code for an STM32F4xx MCU, including a UART for a REPL.  Tested on
a pyboard.  Code size is 77592 bytes.

1 files changed, 20 insertions, 0 deletions

diff --git a/minimal/uart_core.c b/minimal/uart_core.c
index 48cc27d6d..d2d17b4d1 100644
--- a/minimal/uart_core.c
+++ b/minimal/uart_core.c
@@ -5,12 +5,25 @@
  * Core UART functions to implement for a port
  */
 
+#if MICROPY_MIN_USE_STM32_MCU
+typedef struct {
+    volatile uint32_t SR;
+    volatile uint32_t DR;
+} periph_uart_t;
+#define USART1 ((periph_uart_t*)0x40011000)
+#endif
+
 // Receive single character
 int mp_hal_stdin_rx_chr(void) {
     unsigned char c = 0;
 #if MICROPY_MIN_USE_STDOUT
     int r = read(0, &c, 1);
     (void)r;
+#elif MICROPY_MIN_USE_STM32_MCU
+    // wait for RXNE
+    while ((USART1->SR & (1 << 5)) == 0) {
+    }
+    c = USART1->DR;
 #endif
     return c;
 }
@@ -20,5 +33,12 @@ void mp_hal_stdout_tx_strn(const char *str, mp_uint_t len) {
 #if MICROPY_MIN_USE_STDOUT
     int r = write(1, str, len);
     (void)r;
+#elif MICROPY_MIN_USE_STM32_MCU
+    while (len--) {
+        // wait for TXE
+        while ((USART1->SR & (1 << 7)) == 0) {
+        }
+        USART1->DR = *str++;
+    }
 #endif
 }