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140 somewhat on the layout (e.g. stray capacitance). Values in the range of 22pF to 27pF have been used successfully in various situations. ZX-44 Series Clock Source If you already have a clock source at the required frequency you can feed that signal directly to pin 8. In this case there would be no connection to pin 7. Program Memory The ZX-44 series devices (other than the ZX-44n) also require a serial EEPROM in which to store your program’s code. (The ZX-40n uses internal Flash memory for Program Memory.) The recommended device to use is the Atmel AT25256A (Digi-Key #AT25256A-10PI-2.7-ND). Note, particularly, that the older part without the A suffix will not work due to its slower speed. An EEPROM capable of operation at 7.5MHz or more is required. The recommended connection to the ZX is shown below. The value of the pullup resistor is non-critical. Anything from 4.7K to 22K should work well. Higher values will reduce power consumption somewhat. ZX-44 Series Program Memory (VM devices only) With firmware versions v1.1 and later, the Atmel 25HP512 (Digi-Key #AT25HP512-10PI2.7-ND) can also be used for applications that require larger EEPROM space. The electrical connections for this device are identical to those shown above. However, since this device has different characteristics than the AT25256A, special configuration is required. See Section 8.2 for more information.