arduinoattinyspi
I want to use an attiny85 for a project that will be controlling some LEDs over a pseudo SPI that only needs to use SCLK and MOSI. Is there any way to reconfigure pin 6 (MISO) on the 85 to be used as a digital read/write after configuring SPI? I need 3 digital read pins as well as the 2 for SPI and the only way I can d it is either disable the reset pin, which would than mean I would need a high voltage programmer to program it :(, or use the MISO pin…
Any way to do this?
The only difference between a "normal" and "open collector" (or "open drain") output is the addition of an extra transistor (or FET).
Adding an external transistor (with base current limiting resistor) to an output in such a fashion will make it an open-collector output. A single NPN transistor is probably cheaper than a non-inverting buffer if you only need to convert one output. If you are doing multiple outputs then a buffer (say an octal one) would be more sensible from both space and cost points of view.
EDIT
To The simple circuit above will be inverting. One way to make it non-inverting would be to use two transistors:
The first one is the equivalent of the open collector circuit above, which inverts. The second one is the same - open collector that inverts. A pull-up resistor on the first transistor lets it control the second transistor properly. The net effect is a non-inverted open collector conversion.
The number of SPI slaves is not limited. In fact, you can add digital muxes (multiplexors) and control more slave devices than you have digital pins on the Arduino.
However, the SPI was design for communication over short distances within a box. So, the physical size of the bus can become a problem (bus capacitance, EMI). If you have to make a long-distance ruggedized SPI, there are application notes on the subject: Extending the SPI bus for long-distance communication.
Best Answer
The SPI MISO function is nothing but a digital input pin.
The question does not specify whether the pseudo-SPI LED peripheral provides a handshake or other data on the MISO line at all. Assuming there is some initial data at set-up if at all, but no further use of the line by the peripheral during normal operation, the following should work:
Connect one of your 3 data source lines, OR'ed with the MOSI coming from the SPI peripheral, into pin 6 of the ATTiny.
During SPI set-up, ignore (or do what's needed with) the signal coming into pin 6. After that, simply read Pin 6 as you would any normal digital input pin.
If the LED peripheral works fine without connecting it to the MISO on the ATTiny at all, then you can skip the OR connection, and simply treat pin 6 as a standard digital input.