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
QL Add-on ROM
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Re: QL Add-on ROM
I would prefer a wizard, they tend to be more reliable and trustworthyDave 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.
Mark
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“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer in Somerset later in the year
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)
Re: QL Add-on ROM
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?
Tomas
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?
Tomas
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Re: QL Add-on ROM
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
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
Standby alert
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer in Somerset later in the year
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)
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer in Somerset later in the year
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)