Freescale Semiconductor MC68881 - Sense Device (SENSE); Power (VCC and GND)

9.11 SENSE DEVICE (SENSE)
This output pin may be used optionally as an additional GND pin or as an indicator to
external hardware that the FPCP is present in the system. This signal is internally connected
to the GND of the die, but it is not necessary to connect it to the external ground for correct
device operation.
Figure 9-2 shows an example of a circuit to sense the presence of an FPCP in a socket
prepared for it. The circuit asserts BERR when the MPU selects the coprocessor and no
coprocessor is plugged in.
+5 V
I
t0 k
SENSE
SYSTEM
[~ BERR
Figure 9-2. Sense Device Circuit Example
9.12 POWER (Vcc and GND)
These pins provide the supply voltage and system reference level for the internal circuitry
of the FPCP. Care should be taken to reduce the noise level on these pins with appropriate
capacitive decoupling.
The FPCP is fabricated in Freescale's advanced HCMOS process and is capable of operating
at clock speeds of 25 MHz. Although the use of CMOS for a device containing such a large
number of transistors allows significantly reduced power consumption in comparison to
an equivalent NMOS device, the high clock speed makes the characteristics of the power
supplied to the part quite important. The power supply must be as free from noise as
possible, and it mbst be able to supply large amounts of instantaneous current when the
FPCP performs certain operations. In order to meet these requirements, more detailed
attention should be given to the power supply connection to the FPCP than is required for
older NMOS devices that operate at slower clock rates.
In order to provide a solid power supply interface, four VCC pins, eight primary GND pins,
and two secondary GND pins are provided. This allows two VCC and GND pins to supply
the power for the data bus, while the remaining VCC and GND pins are used by the internal
logic and DSACK drivers. The two secondary GND pins are not intended to provide the
main power supply interface, but merely to augment it as required. (One of these pins is
the SENSE pin which may be used as an optional GND connection.)
Three VCC and four GND pin positions are reserved for future use by Freescale and should
be connected appropriately in order to maintain pin compatability with all future versions
of the FPCP. Table 9-4 lists the VCC and GND pin assignments.
In order to reduce the amount of noise in the power supplied to the FPCP, common
capacitive decoupling techniques should be observed. While there is no recommended
MC68881/MC68882 USER'S MANUAL FREESCALE
9-5