QL Real-time clock protection and Battery backup...
Posted: Tue Aug 29, 2017 4:29 pm
Hi everyone
In an unrelated thread, we started to discuss the various solutions to provide Real-time clock protection and Battery backup to a BBQL and whilst its probably old-news for many, I thought I'd break-out the discussion in case there was more to talk about here.
Dave mentioned that he had built Martin Head's Reset Generator cct into a new project, which I too had also tried (but ineffectually - due to my own lack of skills - no reflection on Martin's design!)
Martin's original design can be found archived on Dilwyn's site here:
http://www.dilwyn.me.uk/docs/hardware/index.html
I ended-up replacing the descrete cct with a Reset Generator/Battery backup IC (ADM-692) which more or less does the job of protetcing the clock on the ZX8302 (so-called Peripheral-Chip or 'PC').
Guru Nasta replied:
"The whole point is to pull PCENL to Vcc when power fails and/or reset occurs, as this is how the clock gets corrupt (spurious writes to 8302 registers). It's not the Vbat switchover - this is easy to do using two small schottky diodes. An interestig circuit to look at is the original Qimi PCENL decoder and battery circuit. It is not directly usable with lithium batteries as it incorporates a trickle charger (big no no for lithium!), but it is a clever circuit. It pulls up PCENL as soon as the supply voltage drops below Vbat. Another way to implement (early) power fail detect would be on the 9V input voltage, prior to the power regulator."
As Martin's cct did not attempt to gate PCEN-L, but just provided a clean RESET-L at both power-up and power-down, it suggests that the 8302 might gate PCEN-L itself internally whenever RESET-L goes active.
The chip enable protection would certainly be required for any non-volatile designs built around low-power SRAM (like one of my - almost successful - earlier QL mods), but would be curious to know if it is indeed required for the 8302.
Any further thoughts?
In an unrelated thread, we started to discuss the various solutions to provide Real-time clock protection and Battery backup to a BBQL and whilst its probably old-news for many, I thought I'd break-out the discussion in case there was more to talk about here.
Dave mentioned that he had built Martin Head's Reset Generator cct into a new project, which I too had also tried (but ineffectually - due to my own lack of skills - no reflection on Martin's design!)
Martin's original design can be found archived on Dilwyn's site here:
http://www.dilwyn.me.uk/docs/hardware/index.html
I ended-up replacing the descrete cct with a Reset Generator/Battery backup IC (ADM-692) which more or less does the job of protetcing the clock on the ZX8302 (so-called Peripheral-Chip or 'PC').
Guru Nasta replied:
"The whole point is to pull PCENL to Vcc when power fails and/or reset occurs, as this is how the clock gets corrupt (spurious writes to 8302 registers). It's not the Vbat switchover - this is easy to do using two small schottky diodes. An interestig circuit to look at is the original Qimi PCENL decoder and battery circuit. It is not directly usable with lithium batteries as it incorporates a trickle charger (big no no for lithium!), but it is a clever circuit. It pulls up PCENL as soon as the supply voltage drops below Vbat. Another way to implement (early) power fail detect would be on the 9V input voltage, prior to the power regulator."
As Martin's cct did not attempt to gate PCEN-L, but just provided a clean RESET-L at both power-up and power-down, it suggests that the 8302 might gate PCEN-L itself internally whenever RESET-L goes active.
The chip enable protection would certainly be required for any non-volatile designs built around low-power SRAM (like one of my - almost successful - earlier QL mods), but would be curious to know if it is indeed required for the 8302.
Any further thoughts?