What is fan-out (number of TTL/CMOS inputs) that each Stamp 2 output pin can dr

LittleTyke

How many TTL or CMOS loads can I connect to a Stamp 2 output pin?

Mike Green

It's not an exact number. It depends on the capacitance of the circuit and the speed you want to switch at. Most microprocessor pins can supply about 20-25ma of current and CMOS loads are high impedance and voltage dependent so the issue is that the circuit capacitance needs to be charged quickly enough so the loads will switch on or off in the time available before the next change needs to occur. TTL is current dependent, but very few circuits really use a TTL input. They all use lower current variants.

Realistically, you don't have to worry up to about 10-20 loads at distances up to 1-2 meters with signal rates up to 1-2 MHz. As you get near to those limits, particularly several at once, you may begin to have problems with noise and have to start taking precautions like using twisted pair wiring or something similar, adding termination resistors.

LittleTyke

Thanks, Mike. The additional chips (I haven't decided yet whether TTL or CMOS) will all be on the same circuit board as the Basic Stamp 2. I want to use the Stamp's pins to:

1. Provide a bit address to A, B, C on a 74LS138 3-to-8 decoder
2. Provide a bit address to A0, A1, A2 on several 4099 CMOS latches
3. Provide a 1 or 0 to DATA input on the 4099s
4. Provide a pulse to G1 enable on 74LS138

The Stamp 2 has ample pins to support the above: 3 output pins for #1, 3 pins for #2, 1 pin for #3, 1 pin for #4

The Stamp 2 will receive data from a VB6 program to set the pins accordingly.

Since there will be up to eight 4099s (to provide 64 discrete outputs) the Stamp's pins that achieve #2 above will have to be connected to all eight.

The 74LS138's outputs (inverted) Y0 through Y7 will drive the WRITE DISABLE line on the selected 4099.

T&E Engineer

I did a BS2px project a couple of years back where I had over 110 IC's (4017 and 4081 cascaded) to 700 outputs for LEDs. I used the cascaded 4017 CLOCK and RESET pins that went to the BS2 not to mention an SX28 keypad driver and an LCD display to the BS2px as well.

LittleTyke

T&E Engineer, whew! All I can say is that it looks very impressive indeed!