Added 50kHz operation mode for I2C/Wire library (#3401)

Author: derSeddy

cores/esp8266/core_esp8266_si2c.c

@@ -45,13 +45,15 @@ static uint32_t twi_clockStretchLimit;
 
 void twi_setClock(unsigned int freq){
 #if F_CPU == FCPU80
-  if(freq <= 100000) twi_dcount = 19;//about 100KHz
+  if(freq <= 50000) twi_dcount = 38;//about 50KHz
+  else if(freq <= 100000) twi_dcount = 19;//about 100KHz
   else if(freq <= 200000) twi_dcount = 8;//about 200KHz
   else if(freq <= 300000) twi_dcount = 3;//about 300KHz
   else if(freq <= 400000) twi_dcount = 1;//about 400KHz
   else twi_dcount = 1;//about 400KHz
 #else
-  if(freq <= 100000) twi_dcount = 32;//about 100KHz
+  if(freq <= 50000) twi_dcount = 64;//about 50KHz
+  else if(freq <= 100000) twi_dcount = 32;//about 100KHz
   else if(freq <= 200000) twi_dcount = 14;//about 200KHz
   else if(freq <= 300000) twi_dcount = 8;//about 300KHz
   else if(freq <= 400000) twi_dcount = 5;//about 400KHz
@@ -85,14 +87,16 @@ static void twi_delay(unsigned char v){
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wunused-but-set-variable"
   unsigned int reg;
-  for(i=0;i<v;i++) reg = GPI;
+  for(i=0;i<v;i++) 
+    reg = GPI;
 #pragma GCC diagnostic pop
 }
 
 static bool twi_write_start(void) {
   SCL_HIGH();
   SDA_HIGH();
-  if (SDA_READ() == 0) return false;
+  if (SDA_READ() == 0) 
+    return false;
   twi_delay(twi_dcount);
   SDA_LOW();
   twi_delay(twi_dcount);
@@ -116,8 +120,10 @@ static bool twi_write_stop(void){
 static bool twi_write_bit(bool bit) {
   uint32_t i = 0;
   SCL_LOW();
-  if (bit) SDA_HIGH();
-  else SDA_LOW();
+  if (bit) 
+    SDA_HIGH();
+  else 
+    SDA_LOW();
   twi_delay(twi_dcount+1);
   SCL_HIGH();
   while (SCL_READ() == 0 && (i++) < twi_clockStretchLimit);// Clock stretching
@@ -149,25 +155,30 @@ static bool twi_write_byte(unsigned char byte) {
 static unsigned char twi_read_byte(bool nack) {
   unsigned char byte = 0;
   unsigned char bit;
-  for (bit = 0; bit < 8; bit++) byte = (byte << 1) | twi_read_bit();
+  for (bit = 0; bit < 8; bit++) 
+    byte = (byte << 1) | twi_read_bit();
   twi_write_bit(nack);
   return byte;
 }
 
 unsigned char twi_writeTo(unsigned char address, unsigned char * buf, unsigned int len, unsigned char sendStop){
   unsigned int i;
-  if(!twi_write_start()) return 4;//line busy
+  if(!twi_write_start()) 
+    return 4;//line busy
   if(!twi_write_byte(((address << 1) | 0) & 0xFF)) {
-    if (sendStop) twi_write_stop();
+    if (sendStop) 
+      twi_write_stop();
     return 2; //received NACK on transmit of address
   }
   for(i=0; i<len; i++) {
     if(!twi_write_byte(buf[i])) {
-      if (sendStop) twi_write_stop();
+      if (sendStop) 
+        twi_write_stop();
       return 3;//received NACK on transmit of data
     }
   }
-  if(sendStop) twi_write_stop();
+  if(sendStop) 
+    twi_write_stop();
   i = 0;
   while(SDA_READ() == 0 && (i++) < 10){
     SCL_LOW();
@@ -180,14 +191,18 @@ unsigned char twi_writeTo(unsigned char address, unsigned char * buf, unsigned i
 
 unsigned char twi_readFrom(unsigned char address, unsigned char* buf, unsigned int len, unsigned char sendStop){
   unsigned int i;
-  if(!twi_write_start()) return 4;//line busy
+  if(!twi_write_start()) 
+    return 4;//line busy
   if(!twi_write_byte(((address << 1) | 1) & 0xFF)) {
-    if (sendStop) twi_write_stop();
+    if (sendStop) 
+      twi_write_stop();
     return 2;//received NACK on transmit of address
   }
-  for(i=0; i<(len-1); i++) buf[i] = twi_read_byte(false);
+  for(i=0; i<(len-1); i++) 
+    buf[i] = twi_read_byte(false);
   buf[len-1] = twi_read_byte(true);
-  if(sendStop) twi_write_stop();
+  if(sendStop) 
+    twi_write_stop();
   i = 0;
   while(SDA_READ() == 0 && (i++) < 10){
     SCL_LOW();