Fix 0x00 transmit issue, details see comment

The last commit used the SBI instruction to toggle the TX output pin. If no toggling is required, this instruction was skipped. Executing SBI takes 2 cycles, but skipping only 1 cycle; after 9 consecutive zeros the error was large enough to cause errors on the receiver.

The new code uses the out instruction instead, it always executes in 1 cycle. The flash requirements are identical

Author: nqtronix

main.c

@@ -23,8 +23,8 @@ int main(void)
 
 	_delay_us(40);
 	// toggle pattern test: after every bit delay the TX pin is toggled
-	tinyuart_send_uint8(0b01010101);
- 	_delay_us(10);
+// 	tinyuart_send_uint8(0b01010101);
+//  	_delay_us(10);
 
 	// every character test: verify that your receiver can read all chars at the chosen baud rate
 	// cheap UART-USB converts often have an issue with 0x00
@@ -34,5 +34,9 @@ int main(void)
 		_delay_us(10);
 	}
 
-	while(1);
+	while(1)
+	{
+		tinyuart_send_uint8(0);
+		_delay_us(1000);
+	}
 }

tinyuart.cproj

@@ -43,31 +43,46 @@
         </DebugFile>
       </AAFDebugFiles>
     </AAFDebugger>
-    <avrtool>com.atmel.avrdbg.tool.simulator</avrtool>
-    <avrtoolserialnumber />
+    <avrtool>com.atmel.avrdbg.tool.avrdragon</avrtool>
+    <avrtoolserialnumber>00A200050668</avrtoolserialnumber>
     <avrdeviceexpectedsignature>0x1E9007</avrdeviceexpectedsignature>
     <com_atmel_avrdbg_tool_simulator>
-      <ToolOptions xmlns="">
+      <ToolOptions>
         <InterfaceProperties>
         </InterfaceProperties>
+        <InterfaceName>
+        </InterfaceName>
       </ToolOptions>
-      <ToolType xmlns="">com.atmel.avrdbg.tool.simulator</ToolType>
-      <ToolNumber xmlns="">
+      <ToolType>com.atmel.avrdbg.tool.simulator</ToolType>
+      <ToolNumber>
       </ToolNumber>
-      <ToolName xmlns="">Simulator</ToolName>
+      <ToolName>Simulator</ToolName>
     </com_atmel_avrdbg_tool_simulator>
     <AsfFrameworkConfig>
       <framework-data xmlns="">
-  <options />
-  <configurations />
-  <files />
-  <documentation help="" />
-  <offline-documentation help="" />
-  <dependencies>
-    <content-extension eid="atmel.asf" uuidref="Atmel.ASF" version="3.47.0" />
-  </dependencies>
-</framework-data>
+        <options />
+        <configurations />
+        <files />
+        <documentation help="" />
+        <offline-documentation help="" />
+        <dependencies>
+          <content-extension eid="atmel.asf" uuidref="Atmel.ASF" version="3.47.0" />
+        </dependencies>
+      </framework-data>
     </AsfFrameworkConfig>
+    <avrtoolinterface>ISP</avrtoolinterface>
+    <com_atmel_avrdbg_tool_avrdragon>
+      <ToolOptions>
+        <InterfaceProperties>
+          <IspClock>125000</IspClock>
+        </InterfaceProperties>
+        <InterfaceName>ISP</InterfaceName>
+      </ToolOptions>
+      <ToolType>com.atmel.avrdbg.tool.avrdragon</ToolType>
+      <ToolNumber>00A200050668</ToolNumber>
+      <ToolName>AVR Dragon</ToolName>
+    </com_atmel_avrdbg_tool_avrdragon>
+    <avrtoolinterfaceclock>125000</avrtoolinterfaceclock>
   </PropertyGroup>
   <PropertyGroup Condition=" '$(Configuration)' == 'Release' ">
     <ToolchainSettings>

tinyuart/tinyuart.c

@@ -15,7 +15,7 @@
 #define CYCLES_LOOP		3
 
 // min cycles per tx bit
-#define CYCLES_TX	10
+#define CYCLES_TX	9
 
 
 // the c preprocessor does not support floating point. to get a higher accuracy for divisions the
@@ -28,13 +28,13 @@
 // minimum allowed loop value is 1; value 0 is interpreted as 256
 #ifdef TINYUART_OPT_HIGH_ACCURACY
 	// round to nearest cycle
-	#define CYCLES			(((UPSCALE*TINYUART_F_CPU/TINYUART_BAUD-UPSCALE*CYCLES_TX)+UPSCALE/2)/UPSCALE)
+	#define CYCLES			(((UPSCALE*(TINYUART_F_CPU)/(TINYUART_BAUD)-UPSCALE*CYCLES_TX)+UPSCALE/2)/UPSCALE)
 	#define LOOP_DELAY		(CYCLES/CYCLES_LOOP)
 	#define CYCLES_EXTRA	(CYCLES-(LOOP_DELAY*CYCLES_LOOP))
 
 #else
 	// round to nearest 3 cycles
-	#define LOOP_DELAY		((((UPSCALE*TINYUART_F_CPU/TINYUART_BAUD-UPSCALE*CYCLES_TX)/CYCLES_LOOP)+UPSCALE/2)/UPSCALE)
+	#define LOOP_DELAY		((((UPSCALE*(TINYUART_F_CPU)/(TINYUART_BAUD)-UPSCALE*CYCLES_TX)/CYCLES_LOOP)+UPSCALE/2)/UPSCALE)
 	#define CYCLES_EXTRA	0
 #endif
 
@@ -53,7 +53,7 @@
 
 
 // Calculate tolerance
-#define TOLERANCE_PPM				(1000000*TINYUART_BAUD*(LOOP_DELAY*CYCLES_LOOP+CYCLES_TX+CYCLES_EXTRA)/TINYUART_F_CPU-1000000)
+#define TOLERANCE_PPM				(1000000*(TINYUART_BAUD)*(LOOP_DELAY*CYCLES_LOOP+CYCLES_TX+CYCLES_EXTRA)/TINYUART_F_CPU-1000000)
 #define IS_WITHIN_RANGE(x, range)	((x < -range) || (x > range))
 
 #if   defined(TINYUART_OPT_TOLERANCE_ERR) && IS_WITHIN_RANGE(TOLERANCE_PPM, 45000)
@@ -104,7 +104,7 @@ void tinyuart_send_uint8(uint8_t data)
 	(
 			"mov	%[cnt],			%[data]		\n\t"	// use cnt as scratch pad
 
-			"sbi	%[pin],			%[io]		\n\t"	// Setup START bit => moved for cycles bevore delay loop for accurate timing
+			"out	%[pin],			%[mask]		\n\t"	// Setup START bit => moved for cycles bevore delay loop for accurate timing
 
 			"subi	%[cnt],			128			\n\t"	// invert MSB
 			"lsl	%[cnt]						\n\t"	// shift to a) generate pattern with XOR b) place msb in carry		
@@ -125,8 +125,8 @@ void tinyuart_send_uint8(uint8_t data)
 			"dec	__tmp_reg__					\n\t"
 			"brne	send_bit_delay				\n\t"
 
-			"sbrc	%[data],	0				\n\t"   // toggle TX if change is required
-			"sbi	%[pin],		%[io]			\n\t"
+			"sbrc	%[data],		0			\n\t"   // toggle TX if change is required
+			"out	%[pin],			%[mask]		\n\t"
 
 			"ror	%[data]						\n\t"	// rotation to access the 9th bit in carry
 			"dec	%[cnt]						\n\t"
@@ -139,7 +139,6 @@ void tinyuart_send_uint8(uint8_t data)
 		// input values; never write to any of the below
 		: [mask]	"r"(mask),							// let compiler put mask in register
 		  [loops]	"r"((uint8_t)(LOOP_DELAY+1)),		// the 0th loop iteration already decrements, +1 as compensation
-		  [io]		"I"(TINYUART_IO_TX),
 		  [pin]		"I"(_SFR_IO_ADDR(TINYUART_PIN))
 
 		// cobblers: registers modified without compiler already knowing

tinyuart/tinyuart.h

@@ -21,7 +21,7 @@
 #define TINYUART_F_CPU				9600000
 
 // select a baud rate
-#define TINYUART_BAUD				960000
+#define TINYUART_BAUD				500000
 
 // specify pins used
 #define TINYUART_PORT				PORTB