Need help with an odd design construct

Jim Brain brain at
Tue Nov 14 18:51:21 CST 2017

On 11/14/2017 6:32 PM, Brent Hilpert via cctalk wrote:
> Once the cart is pulled the 4008 chip should end up in standby mode - no enables asserted.
The 3K3 ties both !CROM and !CRAM high, and they are both open collector 
outputs on the port.

But, I forgot to put one important detail in the original note.  I so 
apologize.  The original cart has a HM62256LP-12, but it is evident from 
the traces being cut on the cart that this was not even the original 
SRAM.  I do believe the original SRAM was 32kB in size, though.

SInce the original SRAM was not even original, I replaced with the '4008 
for various reasons.
> In standby a CMOS chip like this will appear as a near infinite impedance, so there isn't much voltage dividing going on with a 6.8K R.
> The full battery voltage (minus epsilon) will be across the chip.
Then, why do the 6K8 there?
> The datasheet specs standby current Isb1 at typically 4 uA (50 max).
> Ohm's law will get you an idea of the effective resistance of the chip if you really want to calculate what epsilon is here.
I'll try and calculate it for the HM62256LP, which seems to be .5mA 
under no load:

(2.7/.0005) = 5K4?    That seems incomplete, like I need to also take 
into account that .5mA flows through the 6800 resistor, but that means 
(6800*.0005) = 3.4V, which can't be right.

If I put that into a Voltage Divider (2.7V Vih, R1 = 5400, R2 = 6800, 
output is 1.5V, implying the GND line is at 1.5V relative to the battery...
> Why the circuit might be as you have it with the 6.8K I'm not sure, perhaps for some current limiting or glitch suppression as the cart is pulled out/in.
> Doesn't really matter much as the only other thing on that gnd side is the switches.
And they aren't there on the original.
> Those 4 10K resistors are kind of weird, each 'on' switch is chewing up orders of magnitude more battery current than the chip,
Good point.  I copied the design from the "ROM cart" schematic, where 
the battery is not in place, and the switches were there to choose the 
bank of 32K that would be visible in the address space. I should 
reconsider the schematic, and...
> might make more sense if they went to Vcc where they would be diode blocked when the cart is on battery.
Excellent suggestion.  I will do so.

Jim Brain
brain at

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