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z80a

sjv

Member
Joined
Nov 24, 2006
Messages
37
Location
Nevada County, CA
I could never really find out on the internet if the z80a is TTL,CMOS etc..Which one is it?

I believe it's NMOS. This was fairly common back then. TTL is too power hungry with little benefit over NMOS to use for a CPU. NMOS typcially uses TTL voltage thresholds and is able to drive at least one TTL load.

-steve
 

sjv

Member
Joined
Nov 24, 2006
Messages
37
Location
Nevada County, CA
Can NMOS meet CMOS voltage levels and can it be interfaced with CMOS SRAM?

Most TTL, NMOS and CMOS utilize TTL levels (0 is less than 0.4V, 1 is more than 2.4V for outputs). Some CMOS, like 4000 series, require 0.70 * VDD or greater for a 1. Often pulling up TTL outputs with a resistor to 5V will do the trick.

Usually CMOS ram use TTL levels. Part of learning about electronics is reading and understanding the data sheets. Voltage thresholds are one aspect. Output drive strength is another. You are on the right path by being aware of these parameters!

-steve
 

UncleBinary

Experienced Member
Joined
Sep 25, 2006
Messages
96
Location
Monroe,Michigan
Exactly what parameter should I be aware of?Oh ya thanks for the site but I want to use low cost SRAM and only chips that you can order from www.jameco.com becuase thats where i plan to get the parts.
 

sjv

Member
Joined
Nov 24, 2006
Messages
37
Location
Nevada County, CA
Exactly what parameter should I be aware of?Oh ya thanks for the site but I want to use low cost SRAM and only chips that you can order from www.jameco.com becuase thats where i plan to get the parts.

Jameco is an excellent source for parts. You can easily put together any number of basic computers from parts ordered from them.

TTL defines 4 threshold parameters:
ViH = 2.0V
ViL = 0.8V
VoH = 2.4V
VoL = 0.4V

ViH and ViL are the input threshold to guarantee a High and Low. VoL and VoH are what an output has to drive to. Notice there is a margin of 0.4V on both high and low.

There's also the input loading and output drive. IH and IL define the required input current. An output driver must be able to "source" enough current into an input to drive it high (above 2.4V), or "sink" enough current to pull the input low (below 0.4V). The output drive characteristics may be explicitly stated in the data sheet, or sort of implied in the VoL/VoH spec. I usually see VoL and VoH defined as 0.4V and 2.4V for a given worse case current. The driver could exceed this current, but then VoL or ViH may not meet the TTL spec.

Let me give you a real world example. The Rockwell 65C02 spec shows VoH on most of it's outputs as 2.4V with an output current of 100uA, and a VoL as 0.4V at 1.6mA. This means that an output can source up to 100uA or sink up to 1.6mA and still meet spec. We want to connect an output to a TTL 74LS00 gate. The 74LS00 has an IH of 20uA and an IL of 0.36mA. The 65C02 can drive 5 74LS00 gates high or 4 74LS00 gates low. ie. - it can drive 4 74LS00 inputs.

Learn to read data sheets as these tell you everything you need to know. A good source for data sheets is http://www.alldatasheet.com/ .

-steve
 
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