Zilog Z80 CPU

the famous z80 cpu is used in a lot of retro consoles from the mega drive to even hand helds like the game gear it is good for diagnostics to understand how the z80 should operate at a fundamental level, even if just a little, to allow with diagnosing issues with systems you can take a deep dive yourself and read the datasheet, but i will explain some basics below package sizes and variants the z80 is not just one cpu it comes in many flavours including package size differences, as well as clock speed differences however, the general operation of all of them are the same the most common packages are zilog z0840004 (dip40 package 4mhz) used in the mega drive and the toshiba 84c00am 6 (ssop 40 package 6mhz) used in the game gear notice the pinout is identical just the package size varies block diagram to familiarize yourself with the cpu its good to look at the pin directions, groupings and block diagram power and clock starting with the basics, the z80 requires ground and 5v on its gnd and vcc pins it also requires to be fed a clock typically in the mhz range but it can be drive as low as khz and up to it's maximum frequency for example, the game gear drives the 84c00am 6 with a 3 579mhz signal inputs below are the inputs and basic description, required to setup correctly in order to have the z80 operating and running the description mentions the state it needs to be in to be in normal running operation so the z80 continues its program execution pin name description 24 !wait needs to be 5v 16 !int needs to be 5v 17 !nmi needs to be 5v 26 !reset needs to be 5v 25 !busreq needs to be 5v in short, the !wait tells the cpu to stop and wait (typically while devices talking to it are getting their data/address ready !int interrupts the cpu for any reason, and while it is low the cpu pauses !nmi is similar, but has a higher priority and interrupts faster, as well as resetting the program position to 0066h this is typically used for things like power failure and critical resets !reset simply keeps the entire cpu reset while low, and once high the cpu starts up in its initial configuration if !busreq is low, the z80 puts all the address, data and control pins high impedence allowing another device to control them so if this is low, the cpu is not driving those pins address / data signals after this, if we have the 5v, gnd, clock and all input signals 5v, the z80 should start incrementing the address pins and reading in data (typically your program instructions from a rom chip if not, something is likely off with the z80