24
XMEGA B [DATASHEET]
8291B–AVR–01/2013
the CPU can access the EEPROM memory while the DMA controller is transferring data from internal SRAM to I/O
memory. The USB module acts as a bus master, and is connected directly to internal SRAM through a pseudo-dual-port
(PDP) interface.
Figure 4-3.
Bus access.
4.9.1
Bus Priority
When several masters request access to the same bus, the bus priority is in the following order (from higher to lower
priority):
1.
Bus Master with ongoing access.
2.
Bus Master with ongoing burst.
1. Alternating DMA controller read and DMA controller write when they access the same data memory section.
3.
Bus Master requesting burst access.
1. CPU has priority.
4.
Bus Master requesting bus access.
1. CPU has priority.
4.10
Memory Timing
Read and write access to the I/O memory takes one CPU clock cycle. A write to SRAM takes one cycle, and a read from
SRAM takes two cycles. For burst read (DMA), new data are available every cycle. EEPROM page load (write) takes one
cycle, and three cycles are required for read. For burst read, new data are available every second cycle. Refer to the
instruction summary for more details on instructions and instruction timing.
4.11
Device ID and Revision
Each device has a three-byte device ID. This ID identifies Atmel as the manufacturer of the device and the device type. A
separate register contains the revision number of the device.
Peripherals and system modules
Bus matrix
CPU
DMA
RAM
OCD
USART
SPI
Timer /
Counter
TWI
USB
Interrupt
Controller
Power
Management
SRAM
External
Programming
PDI
AVR core
ADC
AC
Crypto
Modules
Event
System
Oscillator
Control
CH0
CH1
Non-Volatile
Memory
EEPROM
Flash
CRC
Real Time
Counter
I/O
LCD
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