When sharing a SPI data line between several ICs and an Arduino, what exactly are the frequency constrains of the CS pin? Can I use a shift register to multiplex the CS pins (thus only requiring one I/O pin, the Shift Register CS, out of the Arduino)? Or does CS must be synced carefully with SCK (in order to avoid garbage if CS is pulled high too "late")
Electronic – arduino – Can several CS pins be multiplexed with a shift register
arduinoshift-registerspi
Related Solutions
How is the shift-register wired? Do you have a 0.1uF bypass capacitor across the power leads close to the IC package?
It sounds to me like a noise issue, particularly since it's only triggered when you have a load on the relays. The fact that it results in the shift-register register-state getting reset makes me think it's a power issue.
Also, how are you wiring the shift-register.
With a 74HC595, you need to:
- Tie the two register clocks together (Pins 11 and 12)
- Pull the master-reset pin high (tie pin 10 to VCC)
- Pull the output-enable line low (tie pin 13 to ground)
Lastly, you need a 0.1 uF bypass capacitor between pin 16 (Vcc) and pin 8 (Gnd).
There are a number of options, most of them discussed in this post. Since the communication is one way you can use:
- Voltage dividers: cheap and easey: 2 resistors. Cannot provide stiff voltage sources unless you lose a lot of power through them
- A diode with a resistor circuit. Same as previous
- Level shifter (something like a 74LVC245)
There are shift registers that do run on 3.3V: depending on how many lines you need, an easier solution might be to shift the Arduino output to 3.3V, and feed that to a 3.3V shift register. This will likely (again, depending on your setup) in fewer lines to be level-shifted.
EDIT: Yes, you will need current-limiting resistors for your LEDs, however, voltage divider != a single resistor. Voltage divider looks like this:
simulate this circuit – Schematic created using CircuitLab
EDIT2: Usually not. You should read the datasheet for the shift register you are planning to use about its various maximum voltages. P.S. When you say "serial pins" you don't mean the Serial TX and Serial RX, right? Usually, one drives shift registers using general purpuse digital IO pins. You can use serial pins for two of those, but usually you need a more pins to drive a shift register (usually at least three in the case of 74HC595). The point is 74HC595 is driven via SPI protocol, not RS-232
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Best Answer
Data sheets are usually pretty clear on the timing issues related to chip select lines so I urge you to use the relevant data sheet for the device. Some devices can be activated at the same point at which the data is sent to them; other devices need clear space between CS going low and the SPI data being applied.
Yes, you can use a shift register to select just one particular device but this shift register may need flushing regularly to ensure that over time it hasn't "accumulated" an extra low that could cause two chips o be selected simultaneously. In the main I'd use either a 3 to 8 line decoder for chip selects or individual GPIO lines for each peripheral chip.