MCF5206 polled UART TX issues

[raybellis]

I'm writing some custom firmware for a 25 year old system based on the MCF5206.

I find myself completely unable to transmit any data from UART1, using polling of the USR register, rather than interrupts.

My UART initialisation code looks like this:

	volatile SIM_UART* u = &_sim_base->UART1;

	u->UCR = UART_UCR_RESET_RECEIVER;
	u->UCR = UART_UCR_RESET_TRANSMITTER;
	u->UCR = UART_UCR_RESET_MODE;
	u->UCR = UART_UCR_RESET_ERROR;
	u->UCR = UART_UCR_RESET_BREAK;

	u->UIPCR_UACR = 0;						// IEC = 0
	u->UISR_UIMR = 0;						// no interrupts
	u->USR_UCSR = 0xdd;						// use timer for rx/tx clock

	// set clock for 31250 baud
	const uint16_t ubg = 33000000 / (32 * 31250);
	u->UBG1 = (ubg >>  & 0xff;
	u->UBG2 = (ubg >> 0) & 0xff;

	u->UMR = 0x13;							// UMR1: 8 bits, no parity
	u->UMR = 0x07;							// UMR2: 1 stop bit

	u->UCR = UART_UCR_TX_ENABLE;

and my transmit code like this:

	volatile SIM_UART* u = &_sim_base->UART1;
	uint32_t count = 0;
	uint8_t led = 0x40;

	*io_u35 = 0x20;

	while ((u->USR_UCSR & UART_USR_TX_READY) == 0) {
		*io_u35 = led;
		if (++count == 50000) {
			count = 0;
			led = 0x40 - led;
		}
	}

	u->URB_UTB = n;

The code inside the UART write loop is just there to flash a front panel LED every 50k times around the loop.  The loop never exits, so it seems TxRdy is never getting asserted.

I found a post relating to the 5213 saying that the UARTs don't work right after a power-on-reset, but the proposed fix (writing to the Reset Control Register) doesn't exist on the 5206.

Am I missing something fundamental about how to use the transmit interface?    

[raybellis]

I've tracked down the problem - it appear to be related to my definition of `_sim_base` with its address specified in my linker script:

_sim_base = 0x20000000;

and imported in C code with:

extern volatile SIM* _sim_base;

It works if I use this instead:

static volatile SIM* _sim_base = (void*)0x20000000;

 
I can't be sure yet, but it appeared that with my original code the system was trying to read the contents of 0x20000000 to find `_sim_base`, rather than treating that as an absolute address in its own right.

[TomE]

Do you have any sample code you can copy? I searched and found AN2168. It shows how to use DMA to transmit, but it includes UART initialisation that you might like to compare with yours.

Tom

[raybellis]

I'm working from the 5206 User Manual and the original firmware for the unit.

My code looks *very* similar to that from AN2168, albeit some of the writes done in a different order.  I'll try new code based on AN2168 shortly.