SPI speed not making sense, seems very slow (MEGA2560)

Projects Networking, Protocols, and Devices
1

Hey All,

I'm trying to use the hardware SPI with also using the fastDigitalWrite (using constant pin names) to try and write to 2 devices on the SPI bus (2 MCP4922). So basically, I am managing my CS for both SPI slaves.

What I do is:
-Set CS_0 low
-Or an uint8_t with another uint8_t
-transfer result on SPI
-or an uint8_t with another uint8_t
-transfer result on SPI
-Set CS_0 high, then set CS_0 low (DAC has 2 channels, so I need to toggle CS to latch 1st DAC 16-bit data)
-Or an uint8_t with another uint8_t
-transfer result on SPI
-or an uint8_t with another uint8_t
-transfer result on SPI
-Set CS_0 high

I do this all again with CS_1 (since I have 2 SPI devices I am writing to.

I set the clock divider to 2 so I would think that sets my speed to 8Mhz. or basically 8Mbps

From what you see above, I have 64 bits that need to be written out, and 8 digitalWrites. At 8Mhz, I would think that would give me each of those 64 bits would take 0.125uS. If I assume worst case for the digitalWrites that would be like 4uS I would think, so overall that's 8uS + 32uS or 40uS total. The scope is showing me 250uS (I am alternating the DAC from high to low) to transition from high to low. I don't understand why I am loosing 210uS.

Rick

2

I answered my own question, and so no one else makes the same mistake, this:

SPI.setClockDivider(2);

DOES NOT =

SPI.setClockDivider(SPI_CLOCK_DIV2);

:slight_smile:

Now I see a change every ~19uS (which means the digital writes are much faster) which comes out to 52k changes per second at the DAC outputs when driving 4 channels on 2 SPI devices. Much better.