Serial Boot EEPROM device differences and recommendations
=========================================================

A variety of '2-wire' (i2c) serial EEPROM devices exist on the market.
This document lists some subtle differences, and recommends
which ones to use as boot EEPROMs with current (and future) Trimedia 
devices.

device			size	write protect		addressing	compatibility
			(Bytes)				protocol	(not all tested)

Atmel 24C01A		128	yes, full array		1 byte		TM1000, 1100, 1300

Philips PCA8581C	128	none			1 byte		TM1000, 1100, 1300

Atmel 24C02		256 	yes, full array		1 byte		TM1000, 1100, 1300

Atmel 24C04		512 	yes, full array		1 byte		TM1000, 1100, 1300

Atmel 24C08		1k	none			1 byte		TM1000, 1100, 1300

Atmel 24C16		2k 	yes, upper half (!)	1 byte		TM1000, 1100, 1300

Philips PCF85116-3	2k	yes, full array		1 byte		TM1000, 1100, 1300

Atmel 24C32		4k	yes, upper 1/4 (!)	2 bytes		----

Atmel 24C64		8k	yes, upper 1/4 (!)	2 bytes		----

Atmel 24C128		16k	yes, full array		2 bytes		----

Atmel 24C256		32k	yes, full array		2 bytes		----

Atmel 24C512		64k	yes, full array		2 bytes		----


Recommendation (TM1000,1100,1300)
---------------------------------

For small EEPROMs (host assisted boot, no L1 boot code), use the Atmel
24C01A, 24C02 or 24C04. They have the right write protect functionality.

For larger EEPROMs, as needed for holding an L1 boot program, use the
Philips PCF85116-3, if it is available. It has full array write protection.
The Atmel 24C08 or 24C16 can accidentally get the first array part erased,
even if WP is pulled high. This will result in boot failures.

Clarification - addressing and compatibility
--------------------------------------------

Trimedia TM1000, 1100 and 1300 perform a dummy 0 byte write to EEPROM
word address 0, followed by a sequential read, to read the entire EEPROM. 
The address protocol used is the '1 byte' addressing scheme, as
follows:
  3 msb's of the word address are transmitted within the slave device address
  8 lsb's of the word address are transmitted as 1 address byte

The dummy 0 byte write initializes the EEPROM's 'last address' register.
The subsequent sequential read then causes the EEPOM to retrun all of its
content starting at address 0.

To address devices larger than 2 kBytes, more than 11 bits of address are
needed. In such devices, a 2 byte word address is used. Trimedia TM1000/1100
and 1300 don't send the appropriate dummy 0 byte write with the '2 byte'
protocol to set such devices to the correct initial address state.
Hence TM1000/1100/1300 are limited to using 2 kByte or less EEPROMs.

Future plans (DVP1)
-------------------

We will investigate support for booting from EEPROMs with 1 byte
as well as 2 byte addressing schemes, suchthat midsize applications
can be direct loaded from EEPROM if needed (e.g. a Flash recovery utility).