Reading a lot of digital I/O.....

[Guest]

Hi All,

I have a need to read the state of about 200 digital inputs with a PIC. Obviously, I need to multiplex them in some way to get all the data into the processor. I was thinking shift registers or something like that. Any other clever tricks come to mind?

Thanks,

Derek

[kender]

My favorite trick: I2C I/O expanders such as PCA9671.
_________________
Read the label, before opening a can of worms.

[dyeatman]

It seems to me the type of input would depend on the speed you require. If speed is no concern then the serial expander proposed by Kender would give you almost unlimited inputs with very few control lines. On the other hand bytewide multiplexors would provide a lot more speed and random access at the expense of control lines.

[Guest]

have a look to I2C 16-I/O Ports Expander MCP23017 from Microchip.

Nilsener

[SET]

As above, Microchip port expanders work well.

[kender]

[treitmey]

Perhaps, with the 8 limit you could mux the SCL line out to 3 ""I2C"" buses.

[dyeatman]

One option is to use the CS pins to select banks of MCP23017s. With one CS line from the PIC up to 16 MCP23017 chips could be used.

[SET]

[treitmey]

dyeatman's CS looks better, now. I have to admit,.. I was thinking this route, but didn't see the CS line till I looked twice.
Note that the MCP23S17 is SPI not i2c ((this is the one with CS))

[ckielstra]

The microchip MCP23017 with 16 ports costs about USD1.58 to USD1.85 at Digikey (depending on package type).

Possible alternatives I found using Google are 20-60 pins from Cypress and a 40 pin expander from NXP.

Does anyone have experience with the Cypress port expanders? These are I2C based devices with up to 60 I/O lines in a single package. Port configuration can be stored into the interal EEPROM eliminating the need to reconfigure the device after each power-up. With up to 127 devices addresses per bus there is no need for multiplexing tricks.

-- The CY8C9520 has 20 expansion I/Os, 3 KB of EEPROM and 4 PWM sources. Digikey 1 piece = USD2.23
-- The CY8C9540 has 40 expansion I/Os, 11 KB of EEPROM and 8 PWM sources. Digikey 1 piece = USD3.26
-- The CY8C9560 has 60 expansion I/Os, 27 KB of EEPROM and 16 PWM sources. Digikey 1 piece = USD5.18


NXP (Philips) has the PCA9505 / PCA9506, a 40 bit I2C port expander. Digikey 1 piece = USD5.40

[SET]

[rnielsen]

If any of the bus lines are going to be mux'd then pull-up resistors will be needed on both sides of the mux chip. If not, the mux'd side will see an extra pulse signal on that bus line when it is unselected, which will throw things all hay-wire.

I made a project where I wanted to place several SHT15 sensors on the same bus. The address, of these sensors, cannot be changed. So, I muxed the I2C bus and found I needed pull-ups on the master side and on each sensor's bus.

Ronald

[Peter273]

Personally I used a 4620 lying around here in a setup using 1 port for databus, 1 port for addresbus, one "read/write" line and one "latch" line to simulate a complete "databus/addressbus with RW-control and datalatch-control". It gave me a full 256 "external ports" which are physically distributed over addon-boards on a proprietary bus, but still leaving me enough "free pins" on the 4620 for pwm-out, rs232, adc etc...
All I needed on the external boards were address-decoders to create the chip-selects for the physical IO chips, the rest was just read-write or latch as needed.