FLASH ON ROM MICROBEES

[someone]

Here's just another historical post.

Self flashable ROM on a microbees is easy!
Just ensure that you use the correct Flash ROM parts.
The mods are dead simple and essentially consist of:
  A. The connection of /WR signal on pin 27 on DIP28 or pin 31 on DIP32 devices.
  B. Reconfiguration of decoding of /CE and /OE signals as required.
  C. For larger capacities, the handling of additional Address line pins (e.g. A13, A14, A15 and above).
  D. The enablement of the Flash ROM Software write protection mechanism.

Ensure have a single 5V voltage compatible part such as:
  Xicor X28HC64 (8K x 8)[DIP28]
  Xicor X27C256 (32K x 8)[DIP28]
  Renesas X28C512, X28C513 (64K x 8) [DIP32]
  Atmel AT29C010 (128K x 8) [DIP32]
  SST SST29EE010 (128K x 8) [DIP32]
  Hynix HY29F040A (512K x 8) [TSOP32/PLCC]
avoiding the complexity of parts requiring a 12V programming voltage. e.g.(SST 29SF512).

When using larger capacity FLASH ROMs on a 8238 coreboard:
  one can remove IC31 74XX138 3-8 decoder
  the disconnection the related gates to select the standard Wordbee ROM
  remove the standard Wordbee ROM
  connect the FLASH ROM's higher order address lines (e.g. A13, A14, A15) directly to IC28 4042's quad clock D latch outputs.
  restore the Wordbee ROM functionality to newly installed flash (by burning it into the appropriate Flash ROM bank).

So with just one X28C512/513 you can get 8 PAK banks or 16 PAK banks with an AT290C10.
(To do the latter you'll need to hook up D3 to pin 14 4042 and pin 1 4042 to A16 of the Flash ROM.)

In-circuit programming of the ROMs is a doddle.
Just look up the Flash ROM's datasheet for its simple and effective Software Programming Algorithm.
Essentially it comprises of sending an initiation sequence, followed by a write and and poll for completion sequence for each byte written.

[Ernest]

When using larger capacity FLASH ROMs on a 8238 coreboard:
  one can remove IC31 74XX138 3-8 decoder
  the disconnection the related gates to select the standard Wordbee ROM
  remove the standard Wordbee ROM
  connect the FLASH ROM's higher order address lines (e.g. A13, A14, A15) directly to IC28 4042's quad clock D latch outputs.
  restore the Wordbee ROM functionality to newly installed flash (by burning it into the appropriate Flash ROM bank).

Having done a lot of work around the idea of one ROM containing all the PAK ROM's myself, I will add a couple of things to watch out for if anyone decides to give this a go.
Even if you don't go for a Flash ROM and just go for a Single large ROM to replace the many.

1) Rather than isolating and redirecting the !CE signal, fit a 4K7 resistor between Pin15 and Pin 4 of the previously vacated IC 31 (74XX138) location. This holds the input of the OR gate low preventing it from floating and the !CE to the new ROM will happen whenever !C000h is selected.

2) IC 15 (Wordbee) Pin 26 has a +5V track going to it, needs to be cut prior to connecting the A13 signal from the 4042 Pin 2 output.

3) IC 15 (Wordbee) Pin 1 also has the +5V track going to it, so PIN 1 needs to be isolated from this track before the A15 signal from the 4042's Pin 11 output, however track integrity is still required to maintain +5V distribution. A simple cut either side of the pin and a bypass jumper will suffice.

Not having gone down the track of In-circuit programming of the Microbee ROM myself. A possible thought, was to place the program for doing this in the upper bank of a 27128 NET ROM (Bank 3) IC20, which to my understanding has never had an official program released to occupy this bank location. The decoding is already there on the MB8328 Rev 6 board & should be straight forward to add onto the earlier board with very little alterations.


Ernest

[RedskullDC]

Hi Someone, Ernest, et al.

I did something like this on the Apple ][ , must be 25 years ago now:

http://redskulldc.50megs.com/Flashrom/Flashrom.html

Source code is in the archive if any use to anyone. (6502, but easy to follow)

74LS273 is a good choice for the high address bits, as a RESET can force all outputs to ZERO.

Then you can guarantee a known environment on power up or reset.

Cheers,
Leslie

[someone]

... 74LS273 is a good choice for the high address bits, as a RESET can force all outputs to ZERO.

Thanks for the great post.

This is usually best practice but not when working in conjunction with microbee battery backed RAM and PAK ROMS where the battery backed 4042 latch contents should remain intact and in force for PAK ROMS such as WORDBEE so the user can continue editing their document after a power down or keyboard reset.
[This is actually becomes an annoyance when not using battery backup and the contents of the PAK register are unknown after a power up which makes the powerup + 'M' feature becomes a lottery for the uninitiated.]

The DRAM coreboard employs a 74LS174 resettable flip flop for its memory map configuration register to boot to a known state. (i.e. BOOT ROM at %8000 et al.)
The Alpha Plus (aka Premium Series) and 256TC baseboards use a 74LS273 for their APLUS video configuration options register to boot to a known state. (i.e. standard video mode)