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Fig.2. Typical connections for
Now to the crucial point – what features
do we get in these chips?
There are 10 registers per port – eight
more than most processors provide. The
datasheet for the MCP23S17 is surprisingly complicated, running to 48 pages. This
is simply because Microchip want to pack
as many features into the device as possible
for the price.
With such a simple chip, much of the
cost is determined by the package. The silicon die also has to be a certain size to
allow all the I/O pin wires to connect to it
– so there is no point trying to save on
device functionality when you have the silicon space anyway! Consequently,
Microchip have thought of as many features and options as will physically fit on
the silicon. Of course, you don’t need to
use all the features, and probably wont. It’s
nice to have the options, however.
Configuration options
The configuration registers break down
into two groups of ten, duplicated between
the two 8-bit ports, plus an overall control
register. The common registers are suffixed
with the letter A or B to indicate which port
it is referring to. We will discuss the registers first, and then how to actually access
them – which is more complicated than
you might expect.
IODIR: This determines whether a pin
is an input or an output, just as the TRIS
register does on a PIC processor.
IPOL: Input polarity. An unusual register, this one; when a bit is set it causes the
corresponding input port bit to be inverted
in the input port register GPIO. It’s difficult to think of a use for this, but the
default setting of the register is all zeros,
having no effect, so you can safely ignore
this register.
GPINTEN: Interrupt-on-change enable.
This register allows you to enable one or
more pins to act as interrupt sources. When
an interrupt occurs it simply causes one of
the interrupt pins to change state. You can
wire this pin to an interrupt pin on your
microcontroller to receive immediate notification of a change of state on an input pin
on the MCP23S17,
useful for keyboard
interfaces.
DEFVAL: This
register works in
conjunction with the
GPINTEN register.
When the interrupton-change feature is
enabled, this specifies the ‘default’
expected value on
the corresponding
input pin. When the
input pin takes on a
value different to
the value in DEFVAL, an interrupt is
generated.
INTCON: This
register also works
in conjunction with
the GPINTEN regMCP23S17
ister. It specifies
whether an interrupt
is generated when an input pin changes
state (toggles) or when it changes to a
value that is different to the default value
specified in DEFVAL.
GPPU: On a per pin basis, this allows an
internal pull-up resistor to be enabled for
the specified pins. This is a great feature
for reducing external components, but bear
in mind that the pull-up value is quite weak
– 100kΩ – and is not accurately defined, so
it could vary considerably between different ICs. By default, the pull-ups are disabled, so make sure you enable them for
any unused I/O pin.
INTF: Interrupt status flag register; this
register will indicate which bit, or bits,
caused an interrupt. The bit will stay set in
the register until the GPIO or INTCAP
register has been read.
INTCAP: Interrupt capture register; this
records the status on the input pins at the
instant at which the interrupt occurred.
This can be very useful for recording data
at the precise instant that the interrupt
occurred. After all, it could take several
milliseconds from detecting an interrupt to
actually reading the status of the input
ports. Handy for interfacing to analogueto-digital ICs perhaps.
GPIO: Port register; reading this register will return the status of the port pins. A
write will cause any output pins to take on
the value specified in the write.
OLAT: Output latch register; this register holds the value of data written to a port,
either through this register or to the GPIO
register. It’s equivalent to the LAT register
on the PIC18F devices.
Everyday Practical Electronics, July 2008
Overall control register
Finally, there is an overall control register called IOCON. There are two copies of
this register in the memory map of the
device. The extra one is there for convenience. Seven bits within this register control overall operation of the chip. These
are:
BANK: Selects whether the port A and
port B registers are grouped separately, or
accessed one after the other. This can help
reduce the time it takes to read data from
the two ports (if BANK = 0). There is little
in the difference, so it is probably best
leaving this at its default value of 0.
MIRROR: determines whether the chip
provides a single interrupt output pin
(MIRROR = 1) or one pin per 8-bit port.
SEQOP: Determines whether the internal address pointer of the device increments by one when an access is made to
the device. Turning off sequential operation (SEQOP = 1) increases the speed at
which a single port can be read (or polled)
since it is not necessary to constantly
resend the device address and command.
For non-speed-critical applications it is
probably better to turn off sequential
addressing to help make your software
more readable.
DISSLW: Slew rate control; this is the
one and only configuration option for the
SPI interface. Enabling this feature
improves the reliability of SPI communication at high speeds or when there are many
devices on the SPI bus.
HAEN: Hardware address pins enable; a
strange option, this one. You can disable
the use of the three address selection pins.
If you disable the option you must still wire
the pins to one of the supply rails, and you
must still specify a value of 000 for the
address bits in the SPI message. Best to
leave this feature enabled.
ODR: Determines whether the interrupt
output pins are push-pull or open drain.
Open drain means that the pin can only
drive the signal low, which means you can
parallel up several open drain signals to the
same pin. If you use open drain, you must
provide a pull-up resistor, since the device
can only pull a signal low.
INTPOL: When the ODR register is set
to push-pull, this register determines
whether the ‘an interrupt has occurred’
state is a high or low level.
Next month
In next month’s article we are going to
concentrate on using this chip as an output
port expander, and so will ignore the interrupt facilities of the device. How you use
the interrupt features is very specific to the
particular application, but if there is interest in this subject then we will cover it in
another article.
Each I/O pin can source up to 25mA,
more than enough to drive an LED. Bear in
mind, however, that the maximum current
the device can manage overall is 150mA,
so don’t drive 16 LEDs at 25mA! The
author finds that most LEDs work quite
acceptably at 6mA to 8mA, so with
care you could drive 16 LEDs without
buffering.
Another great feature of the
MCP23S17 is that it can operate down to
1.8V, which fits nicely with the operating
voltage range of some of the small lowpowered microcontrollers. You can easily
drive a circuit from two AAA cell batteries, or a lithium ‘coin’ cell battery if the
overall power consumption is low
enough. When operating below 3V, however, note that the maximum SPI bus
speed is reduced to 5MHz.
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