Demonstrate basic use of USART serial comms with AVR microcontroller.

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>

diff --git a/.gitignore b/.gitignore
new file mode 100644 (file)
index 0000000..d6c2e45
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+*.elf
+*.hex

diff --git a/Makefile b/Makefile
new file mode 100644 (file)
index 0000000..c968506
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,43 @@
+# -*- Makefile -*-
+
+PROJECT := serial-demo
+DEVICE  := atmega328p
+F_CPU   := 16000000UL
+INC     := -I.
+AVRDUDE := avrdude -c usbasp -p $(DEVICE) -C $(AVR_DIR)\etc\avrdude.conf
+
+CC      := avr-gcc
+LD      := avr-gcc
+OBJCOPY := avr-objcopy
+RM      := del >NUL
+CFLAGS  :=-Wall -Wstrict-prototypes -Wmissing-prototypes -Wcast-align -Wshadow \
+          -std=gnu99 -fshort-enums -pedantic-errors -Os -mcall-prologues \
+          -mmcu=$(DEVICE) -DF_CPU=$(F_CPU)
+LDFLAGS := -Wall -mmcu=$(DEVICE)
+
+V := @
+Q := $(V:1=)
+QUIET_CC      = $(Q:@=@echo   CC      $@ &)$(CC)
+QUIET_LD      = $(Q:@=@echo   LD      $@ &)$(LD)
+QUIET_OBJCOPY = $(Q:@=@echo   OBJCOPY $@ &)$(OBJCOPY)
+QUIET_AVRDUDE = $(Q:@=@echo   AVRDUDE $@ &)$(AVRDUDE)
+
+all: $(PROJECT).hex
+
+%.hex: %.elf
+       $(QUIET_OBJCOPY) -j .text -j .data -O ihex $< $@
+    
+%.elf: %.o
+       $(QUIET_LD) $(LDFLAGS) $< -o $@
+    
+%.o: %.c
+       $(QUIET_CC) $(CFLAGS) $(INC) -c $<
+
+flash: $(PROJECT).hex
+       $(QUIET_AVRDUDE) -U flash:w:$<:i
+
+clean:
+       -@$(RM) $(addprefix $(PROJECT), .elf .hex)
+
+.PHONY: clean
+.SECONDARY: $(addsuffix .elf, $(PROJECT))

diff --git a/serial-demo.c b/serial-demo.c
new file mode 100644 (file)
index 0000000..1432c5d
--- /dev/null
+++ b/serial-demo.c
@@ -0,0 +1,85 @@
+/* serial-demo.c - Copyright (c) 2015 Pat Thoyts
+ *
+ * Simple synchronous serial echo firmware for AVR without using stdio.
+ */
+ 
+#include <avr/io.h> 
+#include <avr/power.h>
+
+void usart_init(int baud);
+inline uint8_t usart_is_rx_ready(void);
+inline uint8_t usart_is_tx_ready(void);
+int usart_getchar(void);
+int usart_putchar(char c);
+
+void usart_init(int baud)
+{
+    uint16_t ubrr = F_CPU / 16 / baud - 1;
+    UBRR0H = (uint8_t)(ubrr >> 8); /* write before UBRR0L */
+    UBRR0L = (uint8_t)ubrr;
+    UCSR0B = _BV(RXEN0) | _BV(TXEN0); /* enable RX and TX */
+    UCSR0C = _BV(USBS0) | _BV(UCSZ01) | _BV(UCSZ00); /* async, 8,1,n */
+}
+
+/* nonzero if serial data is available to read. */
+inline uint8_t usart_is_rx_ready(void)
+{
+    return bit_is_set(UCSR0A, RXC0);
+}
+
+/* nonzero if transmit register is ready to receive new data. */
+inline uint8_t usart_is_tx_ready(void)
+{
+    return bit_is_set(UCSR0A, UDRE0);
+}
+
+int usart_getchar(void)
+{
+    loop_until_bit_is_set(UCSR0A, RXC0); /* wait for rx ready */
+    return UDR0;
+}
+
+int usart_putchar(char c)
+{
+    loop_until_bit_is_set(UCSR0A, UDRE0); /* wait until data register empty */
+    UDR0 = c;
+    return 0;
+}
+
+int
+main (void)
+{
+    int n;
+    static char version[] = {'S','e','r','i','a','l',' ','D','e','m','o','\r','\n'};
+
+    /* turn off all unused peripherals */
+    power_adc_disable();
+    power_spi_disable();
+    power_twi_disable();
+
+    /* set all unused pins to high-Z state */
+    DDRB = 0; PORTB = 0xff;
+    DDRC = 0; PORTC = 0xff;
+    DDRD = 0; PORTD = 0xff;
+
+    /* use PB5 (pin 13) as serial receive indicator and trigger */
+    DDRB |= _BV(DDB5); PORTB &= ~_BV(PB5);
+ 
+    usart_init(9600);
+
+    for (n = 0; n < sizeof(version); ++n)
+        usart_putchar(version[n]);
+
+    for (;;)
+    {
+        if (usart_is_rx_ready()) 
+        {
+            char c = 0;
+            PORTB ^= _BV(PB5);
+            c = usart_getchar();
+            usart_putchar(c);
+            PORTB ^= _BV(PB5);
+        }
+    }
+    return 0;
+}