[SOLVED] Can't write to SC1DRL register on 68HC12 board--what am I missing?

Can't write to SC1DRL register on 68HC12 board--what am I missing?

I am trying to write to use the multiple serial interface on the 68HC12 but am can't get it to talk. I think I've isolated the problem to not being able to write to the SC1DRL register (SCI Data Register Low).

The following is from my SCI ISR:

else if (HWRegPtr->SCI.sc1sr1.bit.tdre) {
    /* Transmit the next byte in TX_Buffer. */
    if (TX_Buffer.in != TX_Buffer.out || TX_Buffer.full) {
        HWRegPtr->SCI.sc1drl.byte = TX_Buffer.buffer[TX_Buffer.out];
        TX_Buffer.out++;
        if (TX_Buffer.out >= SCI_Buffer_Size) {
            TX_Buffer.out = 0;
        }
        TX_Buffer.full = 0;
    }
    /* Disable the transmit interrupt if the buffer is empty. */
    if (TX_Buffer.in == TX_Buffer.out && !TX_Buffer.full) {
        Disable_SCI_TX();
    }
}

TX_Buffer.buffer has the right thing at index TX_Buffer.out when its contents are being written to HWRegPtr->SCI.sc1drl.byte, but my debugger doesn't show a change, and no data is being transmitted over the serial interface.

Anybody know what I'm missing?

edit:

HWRegPtr is defined as:

extern HARDWARE_REGISTER *HWRegPtr;

HARDWARE_REGISTER is a giant struct with all the registers in it, and is volatile.

Solution

It's likely that SC1DRL is a write-only register (check the official register docs to be sure -- google isn't turning up the right PDF for me). That means you can't read it back (even with an in-target debugger) to verify your code.

How is HWRegPtr defined? Does it have volatile in the right places to ensure the compiler treats every write through that pointer as something which must happen immediately?