08-10-2020, 07:40 PM
The microbee uses several digital logic signalling thresholds.
As the models progressed and as new logic families introduced, some were used in microbees
For example, LSTTL was replaced with HC CMOS.
This mixture of logic families can have some digital signals looking a bit odd with varying logic high and low levels.
For the original standard microbee, LSTTL was used for the standard system bus however the RESET, POJ, battery backup switchover and related circuits used 74C CMOS logic.
The Z80 CPU clock signal looks a little odd. this is because there is 330 ohm pull up resistor to raise the high logic level to the Z80 CPU specification.
In fact, the use of the 330 ohm pull up resistor on a TTL output is mentioned in the Z80 CPU application notes as a viable circuit.
[Someone's hint: Never trust an Application Note! Test any cited circuits properly to validate whether they are suitable for your design.]
[A useful proverb that someone uses: "There's always more than one way to skin a cat".]
Limitations - The User port port pins are directly connected the Z80 PIO. Be are of the current sink and drive limitations to avoid damaging it.
You'll see many user port device designs use 4.7k ohm series resistors which to the uninitiated seem rather useless.
These are current limiting protection resistors to limit the current to approximately 1mA in case of a fault or intermediate state where the PIO pin is being reconfigured from being an input to an output (and vice versa).
An examples of a user port device that usesthese protection resistors is the Beethoven.
As the models progressed and as new logic families introduced, some were used in microbees
For example, LSTTL was replaced with HC CMOS.
This mixture of logic families can have some digital signals looking a bit odd with varying logic high and low levels.
For the original standard microbee, LSTTL was used for the standard system bus however the RESET, POJ, battery backup switchover and related circuits used 74C CMOS logic.
The Z80 CPU clock signal looks a little odd. this is because there is 330 ohm pull up resistor to raise the high logic level to the Z80 CPU specification.
In fact, the use of the 330 ohm pull up resistor on a TTL output is mentioned in the Z80 CPU application notes as a viable circuit.
[Someone's hint: Never trust an Application Note! Test any cited circuits properly to validate whether they are suitable for your design.]
[A useful proverb that someone uses: "There's always more than one way to skin a cat".]
Limitations - The User port port pins are directly connected the Z80 PIO. Be are of the current sink and drive limitations to avoid damaging it.
You'll see many user port device designs use 4.7k ohm series resistors which to the uninitiated seem rather useless.
These are current limiting protection resistors to limit the current to approximately 1mA in case of a fault or intermediate state where the PIO pin is being reconfigured from being an input to an output (and vice versa).
An examples of a user port device that usesthese protection resistors is the Beethoven.