Fluke 8842a instruction manual In-Guard Microcomputer

Theory of Operation 5

DIGITAL CONTROLLER

5-28. In-Guard Microcomputer

The In-Guard Microcomputer (μC) is a single-chip Z8 microcomputer containing 4K bytes of ROM, 144 bytes of RAM, a UART, and four 8-bit I/O ports. It communicates with the rest of the instrument via the internal bus and dedicated I/O lines. The In-Guard μC is reset when pin 6 is pulled low either by C204 at power-up or by the watch-dog timer in the custom A/D IC (U101). Pin 6 is tied to +5V through a 100 kΩ resistor inside the μC.

All internal bus communication is memory-mapped. Each component that sends or receives data on the bus has a unique address or range of addresses. The internal bus consists of lines AD0-AD7 and A8-A11. Lines AD0-AD7 are time-multiplexed to carry both the least-significant address byte and the data. Lines A8-A11 carry the most- significant bits of the address. The μC writes to and reads from the internal bus according to the read and write cycles shown in Figure 5-14. During either cycle, the address strobe (AS) changes from low to high when an address is valid, and the data strobe (DS) changes from low to high when the data is valid.

The address strobe latches the address on AD0-AD7 into U219 which then provides static address inputs for those devices that need it while data is on the bus. The data memory line (DM) divides the address space between program memory (U222) and data memory (all other devices on the bus). The data memory address space is further divided between the calibration memory (U220) and the remaining devices by A11. The addresses of the remaining devices are decoded from A8-A10 by U208, which combines the address with the data strobe (DS) to provide a chip select (CS0, CS2, CS3, CS4, or CS7) for each device.

The In-Guard μC performs the following functions: range and function control; A/D control and computation; calibration corrections; keyboard/display control; serial communication with the IEEE-488 Interface; and diagnostic self-testing and troubleshooting.

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