SERVICE MANUAL R5888C
QUADRAMHO Chapter 2
Page 14 of 74
4.9.8 General description of microcontroller associated hardware
The scheme logic is implemented using an 8 bit microcontroller operating under
software control. The microcontroller used is the 8039HL member of the MCS 48
family. This device has 27 Input/Output (I/O) lines. Further I/O expansion is
obtained by using three 8255A programmable peripheral interfaces (PPI).
Each PPI has 24 I/O lines made up of 3 x 8 bit ports and is addressed under
software control. An extra six outputs are obtained by using a Hex “D” type flip
flop. A functional diagram of the scheme logic is given in Figure 23. The scheme
logic software is stored in 4k bytes of standard EPROM type D2732A.
The microcontroller operates using a 10.7MHz crystal which gives a cycle time of
approximately 1.40µs.
4.9.9 Microcontroller and program memory
Each byte of program has a specific address within the EPROM. This address has
12 bits and is generated by the microcontroller. The lower 8 bits of the address are
present on the microcontroller’s 8 bit data bus which is multiplexed with other
data. The address is demultiplexed using an 8 bit latch. The higher 4 bits of the
address are present on the lower half of the microcontroller port 2. When the
complete address has been established and latched, the microcontroller reads the
EPROM data and is then able to perform the instruction contained in the data.
4.9.10 Programmable peripheral interface
Each PPI has to be programmed to the required mode of operation before data
can be transferred between the PPI and the microcontroller. Data is written to each
PPI to specify the configuration required.
The configuration used for three PPI is shown below:
PPI 0 has 24 input lines (3 x 8 bit input ports)
PPI 1 has 24 output lines (3 x 8 bit output ports)
PPI 2 has 8 input lines and 16 output lines
(1 x 8 bit input port, 2 x 8 bit output ports)
Data can only be transferred between the microcontroller and any one port of the
PPI at any given instant. Also, because there are three PPI on the data bus (as well
as other devices) only one of these PPI must be enabled at any one time.
Each PPI port (A, B or C) is selected using two address lines, while the PPI is
selected by using another address line to produce the required state on the CS
(chip select) pin. The address is generated by the microcontroller when a read or
write operation is performed. A “D” type flip-flop is used to obtain an extra six
outputs and is addressed in a similar way to the PPI.
4.9.11 Multiplexed port
Port 1 of the microcontroller is an 8 bit port which is used to read the data from the
following:
a) 4 switch banks
b) 2 code switches
c) SOCKET 1
d) 4 other inputs
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