immortalSpirit
Active member
Tutorial on digital I/O, ATMega PIN/PORT/DDR D/B registers vs. ARM GPIO_PDIR / _PDOR A/B/C/D/E registers!
OR, "How to get a fast 8-bit-wide digital read/write!"
I spent the evening figuring this out so I'd like to pass it on to you if u r interested.
My mission: read/write 8 digital I/O pins simultaneously and as fast as possible. I don't care WHICH 8 pins, I just need 8 pins. And I need *fast*.
On the ATMega based Arduino, I used PIND and PORTD, which are each 8 bit registers which read and write pins 0,1,2,3,4,5,6 and 7 packed into a single byte.
On TEENSY-3, the emulation code for PIND and PORTD is very inefficient. Indeed, it is basically 8 digital reads. The code (in avr_emulation.h) basically looks like this:
int ret = 0;
if (digitalReadFast(0)) ret |= (1<<0);
if (digitalReadFast(1)) ret |= (1<<1);
if (digitalReadFast(2)) ret |= (1<<2);
if (digitalReadFast(3)) ret |= (1<<3);
if (digitalReadFast(4)) ret |= (1<<4);
if (digitalReadFast(5)) ret |= (1<<5);
if (digitalReadFast(6)) ret |= (1<<6);
if (digitalReadFast(7)) ret |= (1<<7);
return ret;
That is 8 digitalReadFast's, plus 8 IFs and ORs.
The reason for this is that the actual bits corresponding to the TEENSY-3 pins are scrambled in their order. And therein lies a tale!
After a bunch of reading the "K20 Sub-Family Reference Manual" and studying the TEENSY-3 schematics, it somehow makes sense!
The ARM core has 5 registers much like the ATMel PIN/PORT registers. They are GPIOA, GPIOB, GPIOC, GPIOD, and GPIOE. Each of these supports up to 32 pins. They are *extremely* programmable, and can be mapped in whole or in part to various functions and various pins on the ARM chip.
The TEENSY 3.0 software apparently sets them up as follows:
(where: X = TEENSY-3 external pin number, Y = which GPIO port that reads and writes it, and Z = which bit of that port reads and writes it)
X Y Z
---------------
3 A 12
4 A 13
24 A 5
33 A 4
0 B 16
1 B 17
16 B 0
17 B 1
18 B 3
19 B 2
25 B 19
32 B 18
9 C 3
10 C 4
11 C 6
12 C 7
13 C 5
15 C 0
22 C 1
23 C 2
27 C 9
28 C 8
29 C 10
30 C 11
2 D 0
5 D 7
6 D 4
7 D 2
8 D 3
14 D 1
20 D 5
21 D 6
26 E 1
31 E 0
That's confusing. I'll explain the first four lines:
3 A 12
4 A 13
24 A 5
33 A 4
These say that:
GPIOA bit 12 is connected to pin 3 on the TEENSY-3 circuit board,
GPIOA bit 13 is connected to pin 4,
GPIOA bit 5 is connected to pin 24 (which is on the backside of the board), and
GPIOA bit 4 is connected to pin 33 (also on the backside of the board).
And the 5th line,
0 B 16
says that GPIOB bit 16 connects to pin 0 on the TEENSY-3.
In other words, we are using only 4 of the possible 32 bits of GPIOA, and they are connected to 4 pins of the TEENSY-3, and in scrambled order! In similar manner, GPIOB has 6 of its bits (16,17,0,1,3,2,19 and 18) connected to 6 of the TEENSY-3 external pins (0,1,16,17,18,19, and 25).
If you sort the above table in order by first column (external pin number) instead of second column you can see just how scrambled the pins are:
Teensy-3 Pin, GPIO Port, GPIO Bit #
---------------
0 B 16
1 B 17
2 D 0
3 A 12
4 A 13
5 D 7
6 D 4
7 D 2
8 D 3
9 C 3
10 C 4
11 C 6
12 C 7
13 C 5
14 D 1
15 C 0
16 B 0
17 B 1
18 B 3
19 B 2
20 D 5
21 D 6
22 C 1
23 C 2
24 A 5
25 B 19
26 E 1
27 C 9
28 C 8
How did I determine this table? Two answers: first, look at the TEENSY-3 circuit diagram: http://www.pjrc.com/teensy/schematic.html
Going down the right side of the big chip, we see:
PT C2 45 23/A9
PT C1 44 22/A8
PT D6 63 21/A7
and so on. This corresponds to the lines in my table that read:
23 C 2
22 C 1
21 D 6
(The 45,44, and 63 are irrelevant in this discussion, but are the pin numbers of the physical ARM chip. Just to further confuse you!)
So, a few bits from each of the 5 ports are being used, some more than others, and all in scrambled order!
The other way to determine this table, and corroborating my observations, is the #defines inside the core_pins.h file. Actually, that is where I started from, then noticed the schematic had the same data.
Armed with this understanding, the solution becomes apparent. Let's look just at the lines from the above table which use GPIO "D", and sort them by GPIOD bit number:
2 D 0
14 D 1
7 D 2
8 D 3
6 D 4
20 D 5
21 D 6
5 D 7
In other words, the lower 8 bits of GPIOD map to 8 pins, just like ATMega's PIND/PORTD registers, except that where PIND/PORTD map to pins 7,6,5,4,3,2,1,0 in that order, GPIOD maps to pins 5,21,20,6,8,7,14,2, in that order!
Solution:
1. connect my 8 wires to the TEENSY-3 circuit board pins labeled digital 5,21,20,6,8,7,14,2, IN THAT ORDER. The other ends of these 8 wires goes my own connector labeled 7,6,5,4,3,2,1,0.
2. Execute this statement in my code:
byte allEight = GPIOD_PDIR & 0xFF;
QED! allEight bits 0 through 7 correspond to the data on my wires 0 through 7!
Thus, by choosing this particular set of 8 pins, WE HAVE THE EXACT EQUIVALENT of what the ATMel processors call "PIND".
Note also that I could have chosen port C and gotten TWELVE consecutive pins! Executing this statement:
byte anotherEight = GPIOC_PDIR & 0xFF;
then gives us what ATMel processors call "PINB", scrambled to pins 15,22,23,9,10,13,11 and 12 in that order!
GPIOx_PDIR is the input register for port x, and GPIOx_PDOR is the output register for port x. I won't go into how one can set the direction, it is a bit more complex. Since my application only needs the speed for the read/write, I can use the normal "pinMode" calls to set the direction.
============================
I ran and tested the following code:
// input test:
byte pinTable[] = {2,14,7,8,6,20,21,5};
void setup() {
Serial.begin(0);
for (int i=0; i<8; i++) { pinMode(pinTable,INPUT_PULLUP); }
}
void loop() {
byte eight,prev_eight;
do {
eight = GPIOD_PDIR & 0xFF;
if (eight != prev_eight)
{
prev_eight = eight;
Serial.println(eight,HEX);
}
} while (1==1);
}
With nothing touching the above program prints "FF", all ones. By connecting a jumper from ground and alternately touching it to any of the pins in the "pinTable" above, it printed a new value with the given port bit 0.
And the following program tests output. I took an LED+resistor to ground and touched the other end alternately to one of the specified pins, and within 8 seconds, when the program came around to printing that pin's number, the LED lit!
// output test:
byte pinTable[] = {2,14,7,8,6,20,21,5};
void setup() {
Serial.begin(0);
for (int i=0; i<8; i++) { pinMode(pinTable,OUTPUT); }
}
void loop() {
do {
for (int i=0; i<=7; i++)
{
byte b = 1<<i;
GPIOD_PDOR = b;
Serial.println(pinTable);
delay(1000);
}
} while (1==1);
}
=======================
Thus, in summary:
One can use "GPIOD_PDIR" and "GPIOD_PDOR" as almost exact replacements for "PIND" and "PORTD", if you are willing to use a funny set of pins instead of a pins 0 through 7.
=======================
End of tutorial. Thanks for listening...
OR, "How to get a fast 8-bit-wide digital read/write!"
I spent the evening figuring this out so I'd like to pass it on to you if u r interested.
My mission: read/write 8 digital I/O pins simultaneously and as fast as possible. I don't care WHICH 8 pins, I just need 8 pins. And I need *fast*.
On the ATMega based Arduino, I used PIND and PORTD, which are each 8 bit registers which read and write pins 0,1,2,3,4,5,6 and 7 packed into a single byte.
On TEENSY-3, the emulation code for PIND and PORTD is very inefficient. Indeed, it is basically 8 digital reads. The code (in avr_emulation.h) basically looks like this:
int ret = 0;
if (digitalReadFast(0)) ret |= (1<<0);
if (digitalReadFast(1)) ret |= (1<<1);
if (digitalReadFast(2)) ret |= (1<<2);
if (digitalReadFast(3)) ret |= (1<<3);
if (digitalReadFast(4)) ret |= (1<<4);
if (digitalReadFast(5)) ret |= (1<<5);
if (digitalReadFast(6)) ret |= (1<<6);
if (digitalReadFast(7)) ret |= (1<<7);
return ret;
That is 8 digitalReadFast's, plus 8 IFs and ORs.
The reason for this is that the actual bits corresponding to the TEENSY-3 pins are scrambled in their order. And therein lies a tale!
After a bunch of reading the "K20 Sub-Family Reference Manual" and studying the TEENSY-3 schematics, it somehow makes sense!
The ARM core has 5 registers much like the ATMel PIN/PORT registers. They are GPIOA, GPIOB, GPIOC, GPIOD, and GPIOE. Each of these supports up to 32 pins. They are *extremely* programmable, and can be mapped in whole or in part to various functions and various pins on the ARM chip.
The TEENSY 3.0 software apparently sets them up as follows:
(where: X = TEENSY-3 external pin number, Y = which GPIO port that reads and writes it, and Z = which bit of that port reads and writes it)
X Y Z
---------------
3 A 12
4 A 13
24 A 5
33 A 4
0 B 16
1 B 17
16 B 0
17 B 1
18 B 3
19 B 2
25 B 19
32 B 18
9 C 3
10 C 4
11 C 6
12 C 7
13 C 5
15 C 0
22 C 1
23 C 2
27 C 9
28 C 8
29 C 10
30 C 11
2 D 0
5 D 7
6 D 4
7 D 2
8 D 3
14 D 1
20 D 5
21 D 6
26 E 1
31 E 0
That's confusing. I'll explain the first four lines:
3 A 12
4 A 13
24 A 5
33 A 4
These say that:
GPIOA bit 12 is connected to pin 3 on the TEENSY-3 circuit board,
GPIOA bit 13 is connected to pin 4,
GPIOA bit 5 is connected to pin 24 (which is on the backside of the board), and
GPIOA bit 4 is connected to pin 33 (also on the backside of the board).
And the 5th line,
0 B 16
says that GPIOB bit 16 connects to pin 0 on the TEENSY-3.
In other words, we are using only 4 of the possible 32 bits of GPIOA, and they are connected to 4 pins of the TEENSY-3, and in scrambled order! In similar manner, GPIOB has 6 of its bits (16,17,0,1,3,2,19 and 18) connected to 6 of the TEENSY-3 external pins (0,1,16,17,18,19, and 25).
If you sort the above table in order by first column (external pin number) instead of second column you can see just how scrambled the pins are:
Teensy-3 Pin, GPIO Port, GPIO Bit #
---------------
0 B 16
1 B 17
2 D 0
3 A 12
4 A 13
5 D 7
6 D 4
7 D 2
8 D 3
9 C 3
10 C 4
11 C 6
12 C 7
13 C 5
14 D 1
15 C 0
16 B 0
17 B 1
18 B 3
19 B 2
20 D 5
21 D 6
22 C 1
23 C 2
24 A 5
25 B 19
26 E 1
27 C 9
28 C 8
How did I determine this table? Two answers: first, look at the TEENSY-3 circuit diagram: http://www.pjrc.com/teensy/schematic.html
Going down the right side of the big chip, we see:
PT C2 45 23/A9
PT C1 44 22/A8
PT D6 63 21/A7
and so on. This corresponds to the lines in my table that read:
23 C 2
22 C 1
21 D 6
(The 45,44, and 63 are irrelevant in this discussion, but are the pin numbers of the physical ARM chip. Just to further confuse you!)
So, a few bits from each of the 5 ports are being used, some more than others, and all in scrambled order!
The other way to determine this table, and corroborating my observations, is the #defines inside the core_pins.h file. Actually, that is where I started from, then noticed the schematic had the same data.
Armed with this understanding, the solution becomes apparent. Let's look just at the lines from the above table which use GPIO "D", and sort them by GPIOD bit number:
2 D 0
14 D 1
7 D 2
8 D 3
6 D 4
20 D 5
21 D 6
5 D 7
In other words, the lower 8 bits of GPIOD map to 8 pins, just like ATMega's PIND/PORTD registers, except that where PIND/PORTD map to pins 7,6,5,4,3,2,1,0 in that order, GPIOD maps to pins 5,21,20,6,8,7,14,2, in that order!
Solution:
1. connect my 8 wires to the TEENSY-3 circuit board pins labeled digital 5,21,20,6,8,7,14,2, IN THAT ORDER. The other ends of these 8 wires goes my own connector labeled 7,6,5,4,3,2,1,0.
2. Execute this statement in my code:
byte allEight = GPIOD_PDIR & 0xFF;
QED! allEight bits 0 through 7 correspond to the data on my wires 0 through 7!
Thus, by choosing this particular set of 8 pins, WE HAVE THE EXACT EQUIVALENT of what the ATMel processors call "PIND".
Note also that I could have chosen port C and gotten TWELVE consecutive pins! Executing this statement:
byte anotherEight = GPIOC_PDIR & 0xFF;
then gives us what ATMel processors call "PINB", scrambled to pins 15,22,23,9,10,13,11 and 12 in that order!
GPIOx_PDIR is the input register for port x, and GPIOx_PDOR is the output register for port x. I won't go into how one can set the direction, it is a bit more complex. Since my application only needs the speed for the read/write, I can use the normal "pinMode" calls to set the direction.
============================
I ran and tested the following code:
// input test:
byte pinTable[] = {2,14,7,8,6,20,21,5};
void setup() {
Serial.begin(0);
for (int i=0; i<8; i++) { pinMode(pinTable,INPUT_PULLUP); }
}
void loop() {
byte eight,prev_eight;
do {
eight = GPIOD_PDIR & 0xFF;
if (eight != prev_eight)
{
prev_eight = eight;
Serial.println(eight,HEX);
}
} while (1==1);
}
With nothing touching the above program prints "FF", all ones. By connecting a jumper from ground and alternately touching it to any of the pins in the "pinTable" above, it printed a new value with the given port bit 0.
And the following program tests output. I took an LED+resistor to ground and touched the other end alternately to one of the specified pins, and within 8 seconds, when the program came around to printing that pin's number, the LED lit!
// output test:
byte pinTable[] = {2,14,7,8,6,20,21,5};
void setup() {
Serial.begin(0);
for (int i=0; i<8; i++) { pinMode(pinTable,OUTPUT); }
}
void loop() {
do {
for (int i=0; i<=7; i++)
{
byte b = 1<<i;
GPIOD_PDOR = b;
Serial.println(pinTable);
delay(1000);
}
} while (1==1);
}
=======================
Thus, in summary:
One can use "GPIOD_PDIR" and "GPIOD_PDOR" as almost exact replacements for "PIND" and "PORTD", if you are willing to use a funny set of pins instead of a pins 0 through 7.
=======================
End of tutorial. Thanks for listening...
Last edited:
