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Fast digital IO

I have a device programmable via an 8 bit digital parallel bus. I would like an Arduino to translate from a parallel 4 bit output of an existing device to 8 bit by intentionally reducing the resuloution. Unfortunately, I can't just use the most significant bits as I want to change the step size. The idea is to use an Arduino that ready the 4 digital input bits and then computs the necessary 8 bit output which it output to the 8 bit bus. However, this need to be as fast as 1 us. Is this possible with an Arduino? If so which board would you recommend? I guess I can't use the digitalWrite command as this will be too slow though. Any suggestions?

Answer*

You can do this with an Uno, provided it has nothing else to do. I would
program it low-level, skipping the Arduino core since, as you said,
`digitalWrite()` would be too slow for this application.

Here is my proposed approach: read the 4-bit input from one port, use a
look-up table to translate it to an 8-bit output, and write that output
to another port. I would use PORTD (pins 0–7) for the output, as it is
the only full 8-bit port on the Uno, then the first bits of PORTB (pins
8–11) for the input:

```c++
/* Translation look-up table (dummy example). */
const uint8_t lut[16] = {
 0x00, 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77,
 0x88, 0x99, 0xaa, 0xbb, 0xcc, 0xdd, 0xee, 0xff
};

int main() {
 DDRB = 0x00; // set port B as input
 DDRD = 0xff; // set port D as output
 for (;;) {
 uint8_t input = PINB & 0x0f; // grab the lowest 4 bits
 PORTD = lut[input]; // set the output
 }
}
```

I compiled and disassembled this code to count the CPU cycles: the main
loop takes 10&nbsp;cycles per iteration. Given that the Uno is clocked
at 16 MHz, this is one update of the output every 0.625&nbsp;µs.

**Edit**: This is a straightforward approach. At the cost of some extra
complexity, it could be optimized down to 7&nbsp;cycles per iteration
(0.438&nbsp;µs), or even 6&nbsp;cycles (0.375&nbsp;µs) in assembly.

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nice answer ... upvote for the effort

jsotola
– jsotola

2023年03月04日 22:01:57 +00:00
Commented Mar 4, 2023 at 22:01
Thank you very much for the detailed answer and testing. One further question out of curiosity: Does this mean that if I go for an Arduino Zero, it will be (48 MHz / 16 MHz) 3x faster and for a Due (84 MHz / 16 MHz) 5x faster?

P. Egli
– P. Egli

2023年03月05日 15:14:39 +00:00
Commented Mar 5, 2023 at 15:14
2

@P.Egli: Re: "if I go for an Arduino Zero [...]": No. These Arduinos use an ARM architecture, which is completely different from the AVR architecture of the Uno. The code above may not be trivially ported to them. AVR is simple enough that you can count the CPU cycles simply by looking at an assembly listing. I doubt the ARM cores of the Zero and the Due are at this level of simplicity.

Edgar Bonet
– Edgar Bonet

2023年03月05日 16:22:25 +00:00
Commented Mar 5, 2023 at 16:22

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