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| « Bypassing Boundary-Scan Devices | ScanExpress JET Peripheral Tests - Part 1 » |
Calculate Flash Programming Speed
09/24/10
Calculate Flash Programming Speed
Introduction
Calculating the theoretical Flash programming speed using boundary-scan can
provide a good estimate for the time it will take and allows us to evaluate how
specific factors will affect programming speed. To follow the Tips to Reduce
Flash Programming Time, we’ll look at how to calculate the theoretical
programming speed, and then use the formula to get a better idea of how
different factors may affect programming speed.
Calculating Theoretical Programming Time
For this discussion, let's assume a 16-bit wide S29GL512P Spansion device using
single-word program mode. While this device supports buffered programming with a
32-word buffer, due to the way boundary-scan accesses the Flash, the difference
between single-word and buffered programming times is often minimal. The time it
takes to scan the chain for each data write—which takes up the bulk of the
programming time—remains the same.
The following equation, pulled from our DFT Guidelines, is commonly used for
calculating the theoretical time that it takes to program Flash memory time
using the boundary-scan interface. This equation assumes ideal conditions and
will show the best programming time that can be achieved.
TCK frequency
Where the parameters are defined as:
Table 1: Flash Programming Parameter Descriptions
Example Calculation
We’ll perform an example calculation using the following conditions:
Table 2: Example Flash Program Speed Calculation Data
The absolute minimum programming time that can be achieved when programming the entire device is:
370 seconds for each megabyte of data isn’t great—that’s over 6 minutes! Hopefully there’s something we can do to improve this speed. Let’s see how the chain length and TCK rate will affect programming speed.
How TCK Rate and Chain Length Affect Programming Speed
Using our equation for calculating program time, we can explore how different factors affect programming speed. First, vary the TCK rate between 1 MHz and 25 MHz. The data is presented in the graph below.
Figure 1: Flash Program Rate vs. TCK Frequency
Note that utilizing the External Write signal cuts the programming time approximately in half—utilizing this feature can often provide dramatically improved performance.
Figure 2: Program vs. Scan Chain Length
Notice that as the scan chain length approaches 650 bits with external write and 300 bits without external write, the boundary-scan programming rate crosses the programming rate based on typical write time. At this point, the boundary-scan Flash programming performance will be similar to the programming rate described by the device data sheet, and boundary-scan will be able to scan faster than the device can program! To prevent data errors, the poll-for-done option will need to be utilized to ensure that the previous program operation has completed before the next scan begins.
Conclusion
Now that we have an intuitive understanding of how chain length and TCK rate affect programming rate, we can put our knowledge into practice. Programming rate too slow? See if the TCK rate can be adjusted for Flash programming, and make sure that the chain is being optimized as much as possible.
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