Microcontrollers with Flash Memories

Many of the microcontrollers that Tekmos makes originally used OTP (one time programmable) EPROM for program storage. EPROM is an older and specialized process, and we have had technical issues in incorporating it on our devices.  Instead, we have purchased 4 MB flash memories, and used a stacked die assembly technique to incorporate it on our designs.  This is invisible to the user from an applications point of view.

This approach has many advantages, but it is not without issues.

The Regulator

Flash memories are 3.3 volt devices, and our microcontrollers are typically 5 volt devices.  This requires us to include a voltage regulator in our design.  This regulator must have a very fast response time, since the flash current goes from microamps to 50 mA within a microsecond.  Our regulator does this, but at a price of about 175 microamps of standby current.  We must also have internal level translators to go between the 3.3 volt flash and the 5 volt microcontroller.

Programming

Our flash is not the same as the original EPROM.  It requires 3.3 volts to program and it also requires a special algorithm to do the programming.  Many of the original parts used from 13 to 18 volts to program the EPROM.  If these voltages are applied to our part, it could be destroyed.  Most 3rd party programmers do not support Tekmos.  We just don't have the customer base to interest them.  As a result, we wind up pre-programming the parts before they are shipped.   

Extra Memory

The flash contains an abundance of memory.  If we need a security bit, we will typically use a whole sector of 512K bits for it.  For our 87C751 microcontroller, it originally came with 2K bytes.  We have given it the full 64K memory space, and still have 448K bytes left over.  The 68XX Devices want to know the top byte, and so we use bond options to go between the various device types.  Thus, the 68HC11E1, 68HC711E9, and 68HC711E20 are all the same device, but with different bond options.  We could even make a 68HC711E60 if anyone needed one.

Flash Availability

The flash market is quite volatile.  We are using 4 MB flash memories because they are the smallest we can buy.  In the future, we may find that the 4 MB flashes are not available, and that the smallest we can get is a 128 MB flash.  If that was the case, we could use it, and throw away 127.5 MB so that we can get our 0.5 MB.

Date

2013-02-03