test: add uart receive check via loopback mode

Author: phsauter

.github/scripts/check_sim.sh

@@ -12,6 +12,7 @@ expected_lines=(
   "\[JTAG\] Halting hart 0"
   "\[JTAG\] Resumed hart 0"
   "\[UART\] Hello World!"
+  "\[UART\] Loopback received: internal msg"
   "\[UART\] Result: 0x8940, Cycles: 0xBD"
   "\[UART\] Tick"
   "\[UART\] Tock"

sw/helloworld.c

@@ -31,6 +31,8 @@ uint32_t isqrt(uint32_t n) {
     return res;
 }
 
+char receive_buff[16] = {0};
+
 int main() {
     uart_init(); // setup the uart peripheral
 
@@ -39,6 +41,22 @@ int main() {
     // wait until uart has finished sending
     uart_write_flush();
 
+    // uart loopback
+    uart_loopback_enable();
+    printf("internal msg\n");
+    sleep_ms(1);
+    for(uint8_t idx = 0; idx<15; idx++) {
+        receive_buff[idx] = uart_read();
+        if(receive_buff[idx] == '\n') {
+            break;
+        }
+    }
+    uart_loopback_disable();
+
+    printf("Loopback received: ");
+    printf(receive_buff);
+    uart_write_flush();
+
     // toggling some GPIOs
     gpio_set_direction(0xFFFF, 0x000F); // lowest four as outputs
     gpio_write(0x0A);  // ready output pattern

sw/lib/inc/print.h

@@ -11,4 +11,4 @@
 extern void putchar(char);
 
 // simple printf with support for %x formatter but no others
-void printf(const char *fmt, ...);
+void printf(char *fmt, ...);

sw/lib/inc/uart.h

@@ -30,11 +30,15 @@
 
 // Register fields
 #define UART_LINE_STATUS_DATA_READY_BIT 0
-#define UART_LINE_STATUS_THR_EMPTY_BIT 5
+#define UART_LINE_STATUS_THR_EMPTY_BIT  5
 #define UART_LINE_STATUS_TMIT_EMPTY_BIT 6
 
 void uart_init();
 
+void uart_loopback_enable();
+
+void uart_loopback_disable();
+
 int uart_read_ready();
 
 void uart_write(uint8_t byte);

sw/lib/src/print.c

@@ -27,7 +27,7 @@ uint8_t format_hex32(char *buffer, uint32_t num) {
     return idx;
 }
 
-void printf(const char *fmt, ...) {
+void printf(char *fmt, ...) {
     va_list args;
     va_start(args, fmt);
     char buffer[12];  // holds string while assembling

sw/lib/src/uart.c

@@ -15,15 +15,27 @@ void uart_init() {
     const uint16_t divisor = UART_DIVISOR(UART_FREQ, UART_BAUD); // Calculate from provided config
     uint8_t dlo = (uint8_t)(divisor);
     uint8_t dhi = (uint8_t)(divisor >> 8);
-    *reg8(UART_BASE_ADDR, UART_INTR_ENABLE_REG_OFFSET) = 0x00;   // Disable all interrupts
-    *reg8(UART_BASE_ADDR, UART_LINE_CONTROL_REG_OFFSET) = 0x80;  // Enable DLAB (set baud rate divisor)
-    *reg8(UART_BASE_ADDR, UART_DLAB_LSB_REG_OFFSET) = dlo;       // divisor (lo byte)
-    *reg8(UART_BASE_ADDR, UART_DLAB_MSB_REG_OFFSET) = dhi;       // divisor (hi byte)
-    *reg8(UART_BASE_ADDR, UART_LINE_CONTROL_REG_OFFSET) = 0x03;  // 8 bits, no parity, one stop bit
-    *reg8(UART_BASE_ADDR, UART_FIFO_CONTROL_REG_OFFSET) = 0xC7;  // Enable & clear FIFO, 14B threshold
+    *reg8(UART_BASE_ADDR, UART_INTR_ENABLE_REG_OFFSET)   = 0x00; // Disable all interrupts
+    *reg8(UART_BASE_ADDR, UART_LINE_CONTROL_REG_OFFSET)  = 0x80; // Enable DLAB (set baud rate divisor)
+    *reg8(UART_BASE_ADDR, UART_DLAB_LSB_REG_OFFSET)      = dlo;  // divisor (lo byte)
+    *reg8(UART_BASE_ADDR, UART_DLAB_MSB_REG_OFFSET)      = dhi;  // divisor (hi byte)
+    *reg8(UART_BASE_ADDR, UART_LINE_CONTROL_REG_OFFSET)  = 0x03; // 8 bits, no parity, one stop bit
+    *reg8(UART_BASE_ADDR, UART_FIFO_CONTROL_REG_OFFSET)  = 0xC7; // Enable & clear FIFO, 14B threshold
     *reg8(UART_BASE_ADDR, UART_MODEM_CONTROL_REG_OFFSET) = 0x20; // Autoflow mode
 }
 
+void uart_loopback_enable() {
+    uart_write_flush();
+    uint8_t mcr = *reg8(UART_BASE_ADDR, UART_MODEM_CONTROL_REG_OFFSET);
+    *reg8(UART_BASE_ADDR, UART_MODEM_CONTROL_REG_OFFSET) = mcr | (1 << 4);
+}
+
+void uart_loopback_disable() {
+    uart_write_flush();
+    uint8_t mcr = *reg8(UART_BASE_ADDR, UART_MODEM_CONTROL_REG_OFFSET);
+    *reg8(UART_BASE_ADDR, UART_MODEM_CONTROL_REG_OFFSET) = mcr & ~(1 << 4);
+}
+
 int uart_read_ready() {
     return *reg8(UART_BASE_ADDR, UART_LINE_STATUS_REG_OFFSET) & (1 << UART_LINE_STATUS_DATA_READY_BIT);
 }