Fluke 5720A Interrupt Controller, Glue Logic, RAM Random-Access Memory, ROM Read-Only Memory

Theory of Operation 2

Digital Section Detailed Circuit Description

2-35. Interrupt Controller

PLD U10 is the priority interrupt controller. The interrupt controller reads incoming interrupts and interrupt control lines, then encodes the highest priority interrupt into the interrupt level for the 68HC000. When the 68HC000 responds to an interrupt request, it asks the interrupt controller for an 8-bit vector that corresponds to the pending interrupt of highest priority. The interrupt controller responds with the 4 LSB’s of the vector according to how it is programmed. The 4 MSB’s are pulled up on resistor network Z1. Table 2-5 shows the interrupts, their priority levels, and vectors.

Table 2-5. CPU Interrupts, Priorities, and Vectors

Interrupt

NMI DOGINTH BRPDRTINT* GCDUARTINT* EXDUARTINT* CLKCALINT* BRPIEEEINT* KEYBRDINT* BPSFAILINT* FANINT* RDY/BSYL No interrupt

Priority Level

7

6

5

5

5

4

4

3

2

0

0

0

Vector (Hex)

-(not used) F4

F6

F7

F8

F5

F9 FA FB

FF (not used) FF (not used)

FF

2-36. Glue Logic

ICs U2, U3, and U9 form the glue logic circuit, which keeps various CPU functions running properly. The four OR gates in U9 and an inverter in U3 use control signals UDS*, LDS*, and R/WR* from the microprocessor to generate control signals WRU*, WRL*, RDL*, and RDU*.

2-37. RAM (Random-Access Memory)

Random-access memory is contained in three pairs of sockets, U19 and U20, U21 and U22, and U40 and U41. These sockets accommodate either 32K x 8 or 128K x 8 static CMOS RAM modules (32KB or 128 KB each). The Calibrator is shipped with U19-U22 installed, using 32K x 8 parts and providing 128 KB of static RAM.

2-38. ROM (Read-Only Memory)

Read-only memory is contained in three pairs of sockets, U15-U16, U17-U18, and U23- U24. These sockets accommodate 27010 EPROMS, 128K x 8 devices (128 KB each). Jumpers allow 256 KB devices to be used in their place. The Calibrator is shipped with U15-U18 installed, providing 512 KB of EPROM.

2-25