Memory Chips - Testing and Results

RAM Test

Cambridge Computer wrote a BBC BASIC program to test RAM.

Unless the memory is good the Z88 will become unstable, the FAIL indicator may appear in the top right-hand corner of the display or inconsistent results will occur when checking the size of the RAM with ( CARD). For example, you may have a 512K Ram card in your Z88 but ( CARD) reports that you only have 128K. Failure of the RAM falls in the following categories.

• Mechanical failure - due to the contacts being dirty or from a dry soldered joint
• Unsuitable memory chip - if a different chip is used it may not have the identical specification to the one previously used.

Running the BASIC program while `tapping' the RAM pack may identify a mechanical failure. Finding other errors can be difficult.

The other sort of memory failure is where certain bits of memory suffer amnesia when you are not reading or writing to the memory.

This is like when you are just storing files in the RAM.

This fault can be found with any program, (zCheck, Zip, Unzip (Z88 Forever) or SqueeZ~88 (Rakewell)) that uses checksums on files. To use it you should:-

1. Run the program initially so that the checksums of the files are calculated. If you are using SqueeZ~88 or Zip, squeeze/Zip a load of big files.
2. Turn off the Z88 and leave it alone for about three to five days.
3. Re-run the program to see if the checksums of the files are different or unsqueeze/Unzip all the files again. If they are, then the RAM where the file was stored is faulty.

The following BBC BASIC program tests 32K, 128K 512K and 1MB Ram packs.

It is what is called a destructive Ram Test which means that all the data that is stored on the RAM will be destroyed.

Download the 4 files zipped, (original instructions, two .CLI files (which do the same thing) and the memory test software from here, and then transfer them to your Z88 with a transfer program. If the CLI file is in an EPROM in slot 3,

There are two ways you can use this program.

1. Just running the BBC BASIC program (Ramcard.bas)
2. Before using the program you should save the program to EPROM as "RAMTEST.BAS" and write the following BOOT.CLI to use with it. This will load and run the program automatically after a HARD RESET is given.

#F
|EF
RAMTEST.BAS~E~E
#B
CHAIN
"RAMTEST.BAS"~E

To use the memory test, follow these steps:-

		Load onto an EPROM the BOOT.CLI and RAMTEST.BAS files.
		Back-up all your files.
		Open the card flap Remove the current RAM card.
		Insert the EPROM in slot 3
		Leaving the card flap open, do a Hard Reset. Close the card flap The BOOT.CLI program will run, the memory test programs will be fetched from the EPROM card and then will start to run.
		When you get the Command line type in the number of K the ram card is. E.g. 1024
		Insert the RAM card to be tested in Slot 1.
		Press the key until you get the "Testing . . . " message. Whilst this is testing, tap underneath the Z88 where the card is installed to expose any mechanical electrical problems with the card connector and/or any dry solder joints in the card itself.
		When the test is finished,
		Remove the EPROM pack and
		Do another Hard Reset.
		To make sure that the memory is alright, load a large text file into Pipedream and see if you can read the whole file.

Supply Voltage Min

2.7

Input High Voltage

0.7 x VCC min

to VCC+0.3

2.2 to VCC+0.3

2.4 to VCC+0.3

2.4 to VCC+0.5

2.2 to VCC + 0.5 V

Input Low Voltage

0.5 to 0.8

-0.5 to 0.8

- 0.2 to 0.6

- 0.2 to 0.6

Output High Voltage

2.4 min

Output Low Voltage

0.4 max

0.4 max

0.4  max

Standby Power 

3.5 to 50

VCC for Data Retention

Chip Enable Access Time tACE 

Output Disable to Output in High-Z tOHZ

18 max 

Output Hold from Address Change tOH

Write Cycle Time tWC

Address Valid to End of Write tAW

35 min 

Chip Enable to End of Write tCW 

35 min

Address Set-up Time tAS 

Write Pulse Width tWP 

Write Recovery Time tWR 

Data to Write Time Overlap tDW 

Data Hold from End of Write Time tDH 

Data Setup to Write End

25 min

Output Active from End of Write tOW 

Write to Output in High-Z tWHZ 

LYONTEK - LY62W10248ML-55LLI - SRAM, 8M, 1MX8, 2.7-5.5V, 44TSOPII

This memory chip should be a 'drop-in' replacement for the BSI chip. TESTED FAILS when loading a file in PipeDream.

Manufacture response:-

We have read the information on web-site provided from customer. Just as customer said, yes, the circuit diagram is pretty straightforward. We can not  find an problem from that. Just we know, the major differences between our parts and BSI's are the design on ESD protection devices, shown as below. We use type 2 on our parts, but BSI uses type 1 without using P-diode,Dp. We wondered  if this could be the issue. Maybe, we need to make some tests on customer's AP board to learn the real situations.

My response :-

Sorry about the delay in replying but I have been comparing the specs between both these chips and running more tests.

Where do we go from here?

I just can’t believe it is a timing issue because the Z80 is running at 3.2768 Mhz. A read or write command takes 3 clock cycles. This gives 915.52734375 nS and the device only needs 55.

How can we test and see what is happening? Have you got any suitable test equipment?

Manufacture response:-

Lyontek really wants to realize what happened. We will do our best to find out the issues as Vic would offer his supports, providing measurement results on application boards.

After reading Vic's information and thinking over, I supposed that the major differences I reported, design on ESD protection diodes, could be the issue. The protection diode may limit the trigger voltage in the input nodes to be less than Vcc+0.6V. Hence, I try to check if there exists any input voltage for Flash Memory which is quite over Vcc. The following comes from datasheet of AM29F080B-90ED, in the page 10. I guess that it could  be an issue on Lyontek's parts as the function of Auto-select Mode is executed because the trigger voltage on A9 is limited by ESD protection diode, less than Vcc+5.6V.