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title: 28C256 Notes
has_children: false
nav_order: 3
---

# Notes on 28C EEPROMs

The 28C series parallel EEPROMS, like the 28C256, support fast block writes and algorithms
to implement Software Data Protection (SDP).  The SDP feature seems to be a leading cause
of problems for people trying to program these chips with Arduino or other homebrew
hardware.  

The first problem many people encounter is that new chips are often locked, even though
the datasheet states that they should ship unlocked.  It isn't clear if the manufacturing
practices have changed or if this might be due to used or counterfeit chips.  In any case,
the chips may need to be unlocked before they can be programmed for the first time.

The unlock is accomplished by sending a sequence of bytes to specific addresses.  Many
people have reported problems with this step when using DIY programmers. Some programmers
may not be writing the SDP sequences quickly enough to successfully unlock the chips and
yet others will report that the same hardware works correctly.  This may be due to
variances in the behavior of chips from different manufacturers.

When writing SDP lock/unlock sequences, the datasheets note that the timing between bytes
must follow the same restrictions as page writes.  In particular, the bytes must be
written within the tBLC (Byte Load Cycle time).  On Atmel parts, this is specified as
150us max, so each write pulse must occur within 150us of the previous write.  The tBLC
value is even shorter on the Xicor and ON Semi datasheets, stating that the writes must
occur within 100us of each other.

In practice, the Xicor chips seem very forgiving of the timing, doing successful SDP and
page write operations even when the tBLC is close to 200us.  Atmel chips, on the other
hand, will refuse to unlock when the timing is outside the acceptable maximum.

# Solution

The TommyProm programmer uses direct port access on the Arduino to control the data bus
and addressing shift register.  This is much faster than doing individual DigitalWrite
calls and allows the unlock and page write code to run comfortably within the tBLC
constraints.  It has been successfully tested with the following 28C256 chips:

* Atmel AT28C256-15PU
* Catalyst/ON Semi CSI CAT28C256P-12
* Xicor X28C256P-15
* Xicor X28C256P-25

The Atmel chips tested included a batch from eBay that may not have been genuine, but they
were unlocked and burned successfully with the TommyProm code.

The capture below shows an unlock command sequence where the tBLC is within 80us for each
byte.

![Unlock Timing](images/Unlock-Timing.png)

# References

1. [Atmel AT28C256 Data Sheet, 0006M–PEEPR–12/09](http://ww1.microchip.com/downloads/en/DeviceDoc/doc0006.pdf)
1. [Xicor, Intersil, Renesas X28C256 Data Sheet, 3855-1.9 8/1/97 T1/C0/D8 EW](https://www.renesas.com/us/en/www/doc/datasheet/x28hc256.pdf)
1. [ON Semiconductor CAT28C256 Data Sheet, CAT28C256/D, December, 2009 − Rev. 6](https://www.onsemi.com/pub/Collateral/CAT28C256-D.PDF)
1. [Parallel EEPROM Data Protection Application Note, Atmel, Rev. 0543C–10/98](http://ww1.microchip.com/downloads/en/AppNotes/DOC0543.PDF)
