Context Save and Reload with Real-Timestamp
For context saving and reloading applications, the embedded Flash is used to store multiple contexts
along with their timestamp before entering standby or sleep low power modes and for reloading them
whenever the system wakes. The embedded Flash may also be used to implement ROM-coded state
machines or to store microcode instructions for an embedded processor or any other storage functionality
required by the system application.
The real-time counter (RTC) is another embedded hardware block in Fusion devices. It is a 40-bit counter
with a 40-bit match register and a match flag. The RTC runs off the 3.3 V input to the Fusion device and
can control the 1.5 V supply to the FPGA fabric, creating a standby mode. The clock input of the RTC is
provided by an external crystal oscillator. The RTC block can be configured as a real-time clock to create
timestamps and wake up the FPGA fabric at a specified time. The contents of the 40-bit counter and
match registers (for timestamp comparison) are accessible from the FPGA fabric.
In the context saving application shown in Figure 2, the RAM and register values to be saved along with
an accompanying real-timestamp, or a version stamp with a count instead of a clock, are temporarily
stored in on-chip SRAM. Data is then written to the Flash memory. Once the data is stored in Flash, the
1.5 V voltage regulator can be disabled, turning off power to the FPGA fabric, Flash memory, RAMs, and
registers, leaving the 3.3 V supply (and RTC) operational. Later, the voltage regulator can turn on at a
predetermined time via the RTC or by an external wake signal. The process is reversed at wake-up, taking
data from Flash memory and loading it back into the FPGA fabric, RAMs, and registers.
and State Dual-Port
Timestamp Machine SRAM
Figure 2 • Simple Representation of Context Save and Reload in Fusion Devices
In addition to many other innovative features, Actel Fusion FPGAs provide standby and sleep modes,
embedded Flash memory, and a real-time counter. These capabilities make Actel Fusion technology an
ideal candidate for implementing context saving/reloading and timestamping applications.
List of Changes
The following table lists critical changes that were made in the current version of the document.
Previous Version Changes in Current Version (51900120-1/6.06*) Page
51900120-0/12.05 The low power modes of operation were updated and clarified. N/A
Note: *The part number is located on the last page of the document.
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