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1348FS–ATARM–13-Apr-06
AT91M40800
8. Peripherals
The AT91M40800 microcontroller peripherals are connected to the 32-bit wide Advanced
Peripheral Bus. Peripheral registers are only word accessible – byte and half-word accesses are
not supported. If a byte or a half-word access is attempted, the memory controller automatically
masks the lowest address bits and generates an word access.
Each peripheral has a 16-Kbyte address space allocated (the AIC only has a 4-Kbyte address
space).
8.1 Peripheral Registers
The following registers are common to all peripherals:
• Control Register – write only register that triggers a command when a one is written to the
corresponding position at the appropriate address. Writing a zero has no effect.
• Mode Register – read/write register that defines the configuration of the peripheral. Usually
has a value of 0x0 after a reset.
• Data Registers – read and/or write register that enables the exchange of data between the
processor and the peripheral.
• Status Register – read only register that returns the status of the peripheral.
• Enable/Disable/Status Registers are shadow command registers. Writing a one in the Enable
Register sets the corresponding bit in the Status Register. Writing a one in the Disable
Register resets the corresponding bit and the result can be read in the Status Register.
Writing a bit to zero has no effect. This register access method maximizes the efficiency of bit
manipulation, and enables modification of a register with a single non-interruptible
instruction, replacing the costly read-modify-write operation.
Unused bits in the peripheral registers are shown as “–” and must be written at 0 for upward
compatibility. These bits read 0.
8.2 Peripheral Interrupt Control
The Interrupt Control of each peripheral is controlled from the status register using the interrupt
mask. The status register bits are ANDed to their corresponding interrupt mask bits and the
result is then ORed to generate the Interrupt Source signal to the Advanced Interrupt Controller.
The interrupt mask is read in the Interrupt Mask Register and is modified with the Interrupt
Enable Register and the Interrupt Disable Register. The enable/disable/status (or mask) makes
it possible to enable or disable peripheral interrupt sources with a non-interruptible single
instruction. This eliminates the need for interrupt masking at the AIC or Core level in real-time
and multi-tasking systems.
8.3 Peripheral Data Controller
The AT91M40800 microcontroller has a 4-channel PDC dedicated to the two on-chip USARTs.
One PDC channel is dedicated to the receiver and one to the transmitter of each USART.
The user interface of a PDC channel is integrated in the memory space of each USART. It con-
tains a 32-bit Address Pointer Register (RPR or TPR) and a 16-bit Transfer Counter Register
(RCR or TCR). When the programmed number of transfers are performed, a status bit indicating
the end of transfer is set in the USART Status Register and an interrupt can be generated.
