80C186EB/80C188EB Microprocessor User's Manual - CEUNES

MATH COPROCESSING12.4.1 Clocking the 80C187The microprocessor and math coprocessor operate asynchronously, and their clock rates may differ.The 80C187 has a CKM pin that determines whether it uses the input clock directly or dividedby two. Direct clocking works up to 12.5 MHz, which makes it convenient to feed the clock inputfrom the microprocessor’s CLKOUT pin. Beyond 12.5 MHz, the 80C187 must use a multiplyby-twoclock input up to a maximum of 32 MHz. The microprocessor and the math coprocessorhave correct timing relationships, even with operation at different frequencies.12.4.2 Processor Bus Cycles Accessing the 80C187Data transfers between the microprocessor and the 80C187 occur through the dedicated, 16-bitI/O ports shown in Table 12-7. When the processor encounters a numerics opcode, it first writesthe opcode to the 80C187. The 80C187 decodes the instruction and passes elementary instructioninformation (Opcode Status Word) back to the processor. Since the 80C187 is a slave processor,the Modular Core processor performs all loads and stores to memory. Including the overhead inthe microprocessor’s microcode, each data transfer between memory and the 80C187 (via the microprocessor)takes at least 17 processor clocks.Table 12-7. 80C187 I/O Port AssignmentsI/O Address Read Definition Write Definition00F8H Status/Control Opcode00FAH Data Data00FCH Reserved CS:IP, DS:EA00FEH Opcode Status ReservedThe microprocessor cannot process any numerics (ESC) opcodes alone. If the CPU encounters anumerics opcode when the Escape Trap (ET) bit in the Relocation Register is a zero and the80C187 is not present, its operation is indeterminate. Even the FINIT/FNINIT initialization instruction(used in the past to test the presence of a coprocessor) fails without the 80C187. If anapplication offers the 80C187 as an option, problems can be prevented in one of three ways:• Remove all numerics (ESC) instructions, including code that checks for the presence of the80C187.• Use a jumper or switch setting to indicate the presence of the 80C187. The program caninterrogate the jumper or switch setting and branch away from numerics instructions whenthe 80C187 socket is empty.• Trick the microprocessor into predictable operation when the 80C187 socket is empty. Thefix is placing pull-up or pull-down resistors on certain data and handshaking lines so theCPU reads a recognizable Opcode Status Word. This solution requires a detailed knowledgeof the interface.12-10