Using a counter to reduce pin counts for SPI slave selection

Question

I've got five SPI SRAM chips, which I want to control off a single Arduino. I've seen setups like this, which share SCLK, MOSI and MISO, with separate SS pins:

^{(image courtesty of Wikipedia)}

However, I'd like to avoid using five pins for separate SS enables. I'm considering using a counter to select the different slaves, so I can cut the count down to 2 pins.

The idea is to use two pins for the counter's direction and clock. The code would look something like this:

int currentSlave = 1; // on setup I'll set the counter to 1

void SelectSlave(int id)
{
  // error checking
  if (id < 1 || id > 5) Serial.writeln("Invalid slave ID passed to SelectSlave.");

  // calculate the ID difference
  int diff = abs(id - currentSlave);
  if (diff == 0) return; // no need to do anything

  if (id > currentSlave) set(CTR_DIRECTION); // increment
  if (id < currentSlave) clear(CTR_DIRECTION); // decrement

  // calculate the number of clock pulses to send to the counter
  int pulses = 0;
  if (id > currentSlave) pulses = (1 << (id - 1)) - (1 << (currentSlave - 1));
  if (id < currentSlave) pulses = (1 << (currentSlave - 1)) - (1 << (id - 1));

  // send the clock pulses
  for(int i = 0; i < pulses; i++)
  {
    set(CTR_CLOCK);
    delay(1);
    clear(CTR_CLOCK);
    delay(1);
  }
}

I have a few questions:

• Are there any issues related to turning multiple SS pins on and off during the interim counting period?
• Can I rely on (most) counters being set to zero when it is first powered on?
• Are there other/better ways to cut pin counts in this kind of setup?

Answer 1

You might consider a serial-in-parallel-out shifter like 74HC164. You need 2 pins. You can generate as many enables as you wish with 2 pins. A 74HC138 decoder would need more pins as the number of outputs increased. A counter would need many many clock pulses to enable only one output at a time in some cases. The 74HC164 shifter needs at most n clock pulses to produce n enables.

Answer 2

The typical way to do this is with a demux or decoder 3 to 8 decoder.

However, in your case you would need 3 signals to produce the 5 chip select lines using this approach(you actually get 8 outputs for 3 inputs).

Your approach has merit also, as long as your not sending data while you are cycling through your chip select options. Most counters have a preset line or some method of clearing them on startup.

Answer 3

I have faced a similar problem in the past, but with only two SPI devices. My solution was to use a PIC12F1822 as an SS switcher. It ran at 8 MIPS using its internal oscillator, and had code that did this function:

if SS input goes low then
    if MOSI is low, then
        set SS_OUT_1 low
     else
        set SS_OUT_2 low
    end if
end if

if SS input goes high then
    set SS_OUT_1 high
    set SS_OUT_2 high
end if

It just looped forever on that. The code was written in about 20 assembler instructions, and had a pretty good latency of about a microsecond IIRC.

You could write some very similar code to allow you to select any number of slaves, using MOSI as an up/down signal, E.G.

bits = 0b11111110

begin loop
    if SS input goes low then
        if MOSI is low, then
            bits <<= 1
        else
            bits >>= 1
        end if

        SS_PORT = bits
    end if

    if SS input goes high then
        SS_PORT = 0b11111111
    end if

end loop

Although you'd have to use a PIC with more pins E.G. PIC12LF1840T48A.