uart: fixing baud divisor calculation to add support for 230400 bps

Added code to configure the UART fractional divisor to improve the
UART timing at higher speeds such as 230400, 460800 and 921600 bps.

Signed-off-by: Dan O'Donovan <dan@emutex.com>

Author: Dan-Lightsource

system/libarc32_edu/drivers/ns16550.c

@@ -82,6 +82,7 @@ INCLUDE FILES: drivers/uart.h
 #define REG_MDC 0x04  /* Modem control reg.       */
 #define REG_LSR 0x05  /* Line status reg.         */
 #define REG_MSR 0x06  /* Modem status reg.        */
+#define REG_DLF 0x30  /* Divisor Latch Fraction.  */
 
 /* equates for interrupt enable register */
 
@@ -203,6 +204,7 @@ INCLUDE FILES: drivers/uart.h
 #define MDC(n) (uart[n].port + REG_MDC * UART_REG_ADDR_INTERVAL)
 #define LSR(n) (uart[n].port + REG_LSR * UART_REG_ADDR_INTERVAL)
 #define MSR(n) (uart[n].port + REG_MSR * UART_REG_ADDR_INTERVAL)
+#define DLF(n) (uart[n].port + REG_DLF * UART_REG_ADDR_INTERVAL)
 
 #define IIRC(n) uart[n].iirCache
 
@@ -247,7 +249,7 @@ void uart_init(int which, /* UART channel to initialize */
 	       )
 {
 	unsigned int oldLevel;     /* old interrupt lock level */
-	uint32_t divisor; /* baud rate divisor */
+	uint32_t divisor, fdivisor, tmp; /* baud rate divisors */
 	uint8_t mdc = 0;
 
 	uart[which].port = init_info->regs;
@@ -257,13 +259,22 @@ void uart_init(int which, /* UART channel to initialize */
 
 	oldLevel = interrupt_lock();
 
+	/* The formula for calculating these baud rate divisors is:
+	 *   baud rate = (Fbase) / (16 * (divisor+(fdivisor/16))
+	 */
+
 	/* calculate baud rate divisor */
 	divisor = (init_info->sys_clk_freq / init_info->baud_rate) >> 4;
+	/* Calculate baud rate fractional divisor from the remainder.
+	 * The result is rounded to the nearest whole number */
+	tmp = init_info->baud_rate >> 4;
+	fdivisor = (((init_info->sys_clk_freq >> 4) % init_info->baud_rate) + (tmp >> 1)) / tmp;
 
 	/* set the DLAB to access the baud rate divisor registers */
 	OUTBYTE(LCR(which), LCR_DLAB);
 	OUTBYTE(BRDL(which), (unsigned char)(divisor & 0xff));
 	OUTBYTE(BRDH(which), (unsigned char)((divisor >> 8) & 0xff));
+	OUTBYTE(DLF(which), (unsigned char)(fdivisor & 0xf));
 
 	/* specify the format of the asynchronous data communication exchange */
 	OUTBYTE(LCR(which), init_info->async_format);