How to use gpio on RCM6700

The following is not specifically for the RCM6700 but it should show you how to proceed:

Note: This information is general to the Rabbit 4000 and certain pins may not be available on your product.

The first thing I want to do is to point you to our Rabbit 4000 Processor Easy Reference Poster:

The poster shows all the registers you need to set and makes it pretty easy to find what you need.

In the center of the poster is a description of the Parallel Ports and the registers associated with each one. For more specific details on the Parallel Ports, you will want to look here:

Using the digital I/O on the Rabbit products isn’t difficult but is easier to do after looking at an example. Let’s step back for a moment and consider a simpler problem like lighting up two LEDs on Parallel Port D.

You could hook the LEDs directly up to PD4 and PD5 and use pull-up resistors with the LEDs like this.

+5 Volts ------////------- LED1 ----------PD4

+5 Volts ------////------- LED2 ----------PD5
(I would recommend a 330 Ohm or bigger resistor.)
With the LEDs hooked up as above, you should be able to use this code to light both LEDs.

#define ON 0
#define OFF 1

WrPortI(PDDCR, &PDDCRShadow, 0); // make D driven high and low (see manual for details)
WrPortI(PDDDR, &PDDDRShadow, 0xFF); // make D output (see manual for details)
BitWrPortI(PDDR, &PDDRShadow, ON, 4); // PD4 becomes GND and LED lights up
BitWrPortI(PDDR, &PDDRShadow, ON, 5); // PD5 becomes GND and LED lights up

PDDCR = Port D Drive Control Register (Use this to set open drain)
PDDDR = Port D Data Direction Register (Use this to define input or output)
PDDR = Port D Data Register (Use this to actually send the data)

This provides a very basic example of the digital I/O of a Rabbit microprocessor.

Full details on Parallel Port D (and more) can be found in the Rabbit 4000 Microprocessor User’s Manual here:

After looking over this example, you should be able to do more complicated things.

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That didn’t show up well. It should have been:

+5V to resistor to LED1 to PD4

+5V to resistor to LED2 to PD5

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Thank You, trooper2, your answer helped me a lot.
The problem is, that I meed to get 32 pins for gpio. Now I know how to use ports: B,C,D,E (28 pins). How can I manage port A?
Many thanks to trooper2 and to other community members.

I try to write to PA, with WrPortI(PADR, NULL, 0xFF), but can’t see any changes on pins.

Hello Gitomjk, you can use port A (the programming port) when you put the unit in run mode. This is used for the programming port and will not when when debugging.

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Parallel Port A is configured byte-wise. That means that all ports are either inputs or outputs and there cannot be a combination. Port A is unique in this respect and the other ports can have inputs and outputs on the same port.

The WrPortI is used to write a value to a register and it needs three parameters.

WrPortI(SPCR, &SPCRShadow, 0x84);

Parameter 1 - The register you wish to change
Parameter 2 - The shadow register of the register you wish to change
Parameter 3 - The value to write to the register

With any Port except Port A, you could also use BitWrPortI to configure a single bit on the port. (Remember that Port A is configured byte-wise.)

BitWrPortI(PGDR, &PGDRShadow, 1, 0);

Parameter 1 - The register you wish to change
Parameter 2 - The shadow register of the register you wish to change
Parameter 3 - The value to write to the register
Parameter 4 - Which bit you wish to change


  1. Does this mean that I cannot use any of the A ports as inputs?

This means you can use all the pins on Port A as inputs OR outputs, but not both.

  1. If I wanted to use pins on port B as well or another port as outputs, what is the equivalent line command like the above that configures port B or another port as output?

Port A looks a little funny because the register is named Slave Port Control Register (SPCR). The other ports use a “Data Direction Register” to set the direction of the data.

Port A - No register to specifically control bits (Use SPCR for byte-wise)
Port B – PBDDR
Port C - PCDDR (Pins used for serial lines)
Port D - PDDDR
Port E - PEDDR

  1. With the above line command in the code, can I intermix inputs and outputs on the same port? For example PA0 = output, PA3=input,….

You won’t be able to use BitWrPortI to set up individual inputs and outputs on Port A as we discussed earlier. When you want to set individual bits to inputs and outputs on a port that will allow that, you can use BitWrPortI to do it.

In the code here I have set the Port E Data Register (PEDR). The value is “1” and the pin number is “0”. This code would continuously toggle the data bit:

while (1)
BitWrPortI(PEDR, &PEDRShadow, 1, 0); // Set PE0 to value 1
BitWrPortI(PEDR, &PEDRShadow, 0, 0); // Set PE0 to value 0

  1. What does the 0x84 mean?

The value is a hexadecimal number and we use that system because it makes it easier to read the bits at a glance. Take a look at this table:

Hex Binary
0 0000
1 0001
2 0010
3 0011
4 0100
5 0101
6 0110
7 0111
8 1000
9 1001
A 1010
B 1011
C 1100
D 1101
E 1110
F 1111

Each hexadecimal digit is 4 bits and two digits gives us a full byte.

Some examples:
0x00 = 0000 0000
0x40 = 0100 0000
0xAC = 1010 1100
0x84 = 1000 0100

This statement WrPortI(SPCR, &SPCRShadow, 0x84) writes “1000 0100” to the bits of the SPCR register.

Looking at the Easy Reference Poster, I look at SPCR in the middle of the poster under SLAVE PORT to see how to set the bits.

Bit 7 - 1 (ignore smode pins)
Bit 6 - 0 (ignored for Write)
Bit 5 - 0 (ignored for Write)
Bit 4 - 0 (Disable slave port - port is byte-wide input)
Bit 3 - 0 (Disable slave port - port is byte-wide input)
Bit 2 - 0 (Disable slave port - port is byte-wide input)
Bit 1 - 0 (Priority 2 interrupt)
Bit 0 - 1 (Priority 2 interrupt)

Hello Trooper and Margaret,
I figured out, how to use all the parallel ports finally, and the app works! Now, I need to get back to debug mode( so I can see console from my dynamic C IDE).
I changed back my SSPCR settings, but it didn’t help.

Thank you ,again,