QL Add-on ROM

Nagging hardware related question? Post here!
User avatar
Pr0f
Super Gold Card
Posts: 620
Joined: Thu Oct 12, 2017 9:54 am

Re: QL Add-on ROM

Postby Pr0f » Tue Mar 31, 2020 7:51 am

You could make use of a DS1210 chip to gate control of your /CE based on power supply readiness - this would protect the chip from write corruption when you switch off the main power.

https://datasheets.maximintegrated.com/en/ds/DS1210.pdf


User avatar
1024MAK
Super Gold Card
Posts: 566
Joined: Sun Dec 11, 2011 1:16 am
Location: Looking forward to summer in Somerset, UK...

Re: QL Add-on ROM

Postby 1024MAK » Tue Mar 31, 2020 9:34 am

Dave wrote:You could have a physical witch to connect /WR. If you tie it to 5V it can’t be written. A mechanical pull-up.

I would prefer a wizard, they tend to be more reliable and trustworthy :lol: :lol: :lol:

Mark


QL, Falcon, Atari 520STFM, Atari 1040STE, more PC's than I care to count and an assortment of 8 bit micros (Sinclair and Acorn)(nearly forgot the Psion's)
tcat
Super Gold Card
Posts: 617
Joined: Fri Jan 18, 2013 5:27 pm
Location: Prague, Czech Republic

Re: QL Add-on ROM

Postby tcat » Tue Mar 31, 2020 9:45 am

Hi,

All the SRAM backed protection, makes me thinking about mechanical bank switching on 512K eeprom, as I can address only 256K upper or lower halves. Here I rely that a reverse leakage is big enough to pull address line high, and not that big to be pulled low with a switch. I know resistors could be used instead, but I am interested in a use of a suitable diode? Could this work on power on as well as runtime?

256k-switch.png
mechanical bank switch

Tomas


User avatar
1024MAK
Super Gold Card
Posts: 566
Joined: Sun Dec 11, 2011 1:16 am
Location: Looking forward to summer in Somerset, UK...

Re: QL Add-on ROM

Postby 1024MAK » Tue Mar 31, 2020 10:33 am

I strongly recommend you do use a resistor to pull up undriven address line inputs. For CMOS chips, a value of between 4.7kΩ and 10kΩ is suitable.

If an address line for a CMOS SRAM is allowed to float, it will pick up interface and static charge from other conductors or conductive bodies/objects. And it could then ‘see’ a logic low or a logic high or keep flipping between high and low. Then the CPU may well access the wrong memory location(s)...

The reverse leakage current of a silicon diode reverse biased at 5V is tiny and may not be enough to be reliable. And in fact, I see no need for a diode as shown on your schematic. Put a resistor in place of the diode ;)

Mark


QL, Falcon, Atari 520STFM, Atari 1040STE, more PC's than I care to count and an assortment of 8 bit micros (Sinclair and Acorn)(nearly forgot the Psion's)
User avatar
Pr0f
Super Gold Card
Posts: 620
Joined: Thu Oct 12, 2017 9:54 am

Re: QL Add-on ROM

Postby Pr0f » Tue Mar 31, 2020 11:06 am

+1 on that - diodes are not good as pull ups !



Who is online

Users browsing this forum: No registered users and 3 guests