Design Ideas: June 22, 1995
When you connect a slow device to a µC, you insert a number of wait states to extend the read or write cycle. For the 8031 or 8051 family, however, no input is available for extending the cycle. So, to extend the cycle, stretch the period of the clock connected to the µC's XTAL1 input. In Fig 1, IC3 is an up/down counter configured as a down counter, with the number of wait states preset at inputs A to D. The preset IC4 enables the clock connected to the µC's XTAL1 input.
When you address IC1 with a low state on either CPU_RD or CPU_WR, pin 3 of IC2 switches high and allows IC3 to start counting down. Pin 6 of IC4 goes high to stretch the clock period. When counter IC3 reaches zero, pin 12 of IC3 goes high and stops the counting. Pin 6 of IC4 switches from high to low at the next low-to-high transition of the clock, thereby reenabling the clock. Certain 8031 or 8051 family members have a minimum requirement on the clock frequency which is not so for the 80CL31 and 80CL51 from Philips. (DI #1718)