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2.13 Exception Processing
2.13.1 Divisor Latch Not Programmed
Since the processor reset signal has no effect on the divisor latch, the divisor latch will have an unknown
value after power up. If the divisor latch is not programmed after power up, the baud clock (BCLK) will not
operate and will instead be set to a constant logic 1 state.
The divisor latch values should always be reinitialized following a processor reset.
2.13.2 Changing Operating Mode During Busy Serial Communication
Since the serial link characteristics are based on how the control registers are programmed, the UART will
expect the control registers to be static while it is busy engaging in a serial communication. Therefore,
changing the control registers while the module is still busy communicating with another serial device will
most likely cause an error condition and should be avoided.
3 Registers
The system programmer has access to and control over any of the UART registers that are listed in
Table 6. These registers, which control UART operations, receive data, and transmit data, are available at
32-bit addresses in the device memory map. See the device-specific data manual for the memory address
of these registers.
• RBR, THR, and DLL share one address. When the DLAB bit in LCR is 0, reading from the address
gives the content of RBR, and writing to the address modifies THR. When DLAB = 1, all accesses at
the address read or modify DLL. DLL can also be accessed with address offset 20h.
• IER and DLH share one address. When DLAB = 0, all accesses read or modify IER. When DLAB = 1,
all accesses read or modify DLH. DLH can also be accessed with address offset 24h.
• IIR and FCR share one address. Regardless of the value of the DLAB bit, reading from the address
gives the content of IIR, and writing modifies FCR.
Table 6. UART Registers
Offset Acronym Register Description Section
0h RBR Receiver Buffer Register (read only) Section 3.1
0h THR Transmitter Holding Register (write only) Section 3.2
4h IER Interrupt Enable Register Section 3.3
8h IIR Interrupt Identification Register (read only) Section 3.4
8h FCR FIFO Control Register (write only) Section 3.5
Ch LCR Line Control Register Section 3.6
10h MCR Modem Control Register Section 3.7
14h LSR Line Status Register Section 3.8
20h DLL Divisor LSB Latch Section 3.9
24h DLH Divisor MSB Latch Section 3.9
28h PID1 Peripheral Identification Register 1 Section 3.10
2Ch PID2 Peripheral Identification Register 2 Section 3.10
30h PWREMU_MGMT Power and Emulation Management Register Section 3.11
21
SPRU997C–December 2009 Universal Asynchronous Receiver/Transmitter (UART)
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