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CHAPTER 9
SLAVE PORT
The slave port offers an alternative for communication between two microcontrollers. Tradition-
ally, design engineers have had three options for achieving this communication — a serial link, a
parallel bus without a dual-port RAM (DPRAM), or a parallel bus with a DPRAM to hold shared
data.
A serial link, the most common method, has several advantages: it uses only two pins from each
device, it needs no hardware protocol, and it allows for error detection before data is stored. How-
ever, it is relatively slow and involves software overhead to differentiate data, addresses, and
commands. A parallel bus increases communication speed, but requires more pins and a rather
involved hardware and software protocol. Using a DPRAM offers software flexibility between
master and slave devices, but the hardware interconnect uses a demultiplexed bus, which requires
even more pins than a simple parallel connection does. The DPRAM is also costly, and error de-
tection can be difficult. The SSIO offers a simple means for implementing a serial link. The mul-
tiplexed address/data bus can be used to implement a parallel link, with or without a DPRAM.
The slave port offers a fourth alternative.
The slave port offers the advantages of the traditional methods, without their drawbacks. It brings
the DPRAM on-chip, inside the microcontroller (Figure 9-1). With this configuration, the exter-
nal processor (master) can simply read from and write to the on-chip memory of the 8XC196NT
(slave) processor. The slave port requires more pins than a serial link does, but fewer than the
number used for a parallel bus. It requires no hardware protocol, and it can interface with either
a multiplexed or a demultiplexed bus. The master CPU simply writes to or reads from the device
as it would write or read any parallel interface device (such as a memory or an I/O port). Data
error detection can be handled through the software.
9-1
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