MCP23S08 doesn't work

[Guest]

Does anyone have a working code example for the MCP23S08 GPIO expander? I can't seem to find what is wrong with the code below. The expander chip never replies to anything sent to it. It's SDO pin stays low all the time. This device is like the MCP23S17 but it only has one I/O port.

//* write_gpio(int8 address, int8 data) subroutine ******************************
// address = register address
// data = data to place in register
// Initializes GPIO chip

void write_gpio(int8 address, int8 data) {

output_low(GPIO_CS);
spi_write(0x40); // Hardware address and R/W bit (bit 6 always on)
spi_write(address); // Register address
spi_write(data); // Data to send
output_high(GPIO_CS);

}
//******************************************************************************


//* read_gpio(int8 address) function *******************************************
// address = register address
// Returns: data from register
// Reads a value from the MCP23S08 GPIO expander

int8 read_gpio(int8 address) {

int8 data=0;
output_low(GPIO_CS);
spi_write(0x41); // Hardware address and R/W bit (bit 6 always on)
spi_write(address); // Register address
data=spi_read(0); // Read data in
output_high(GPIO_CS);
return(data);

}

[PCM programmer]

Post your setup_spi() statement. Also post your PIC type, #fuses
statement, and #use delay() statement.

[Guest]

Here is the setup_spi statement and the first few lines of the source code.

#include "18f452.h"
#device ADC=10
#fuses HS, NOPUT, NOWDT, NOPROTECT, NOLVP, CCP2C1, NODEBUG, NOBROWNOUT
#use delay (clock=3686400)

setup_spi(SPI_MASTER | SPI_L_TO_H | SPI_CLK_DIV_64);

I've played with the SPI clock quite a bit trying different values. Not that it should matter though since the MCP23S08 can run up to 10MHz and my crystal is only good for 3.6864MHz.

[PCM programmer]

Based on the Figure 2-5 (pg. 26) of the MCP23S08 data sheet, "SPI Input
Timing", it looks like data should be clocked out of the PIC on the negative edge of SCLK. This means you need to change the edge
parameter of setup_spi() to:

[Guest]

Thanks. I needed SPI_XMIT_L_TO_H as well, but you certainly got me going in the right direction. I'll post up what I have in the code library when I get this completed as a reference for anyone else who has problem(s) with this family of chips.

[Humberto]

The MCP23S08 has 10 Configuration and Control Registers that determine its behaviour.
You didn't show us how it was setted. To help you we need a complete and compilable
example code that demostrate the problem.


Humberto

[lfander]

What does this comment mean ???

spi_write(0x40); // Hardware address and R/W bit (bit 6 always on)

I've tried using this expander to light LEDs. Seven pins will sink current, but bit 6 will never will. Any ideas

[PCM programmer]

[lfander]

I understand drivers and I did look at the data sheet first. Bit 6 is always high implies a hardware condition, not a "1" in the code sent to the device, thats why I asked. The pin controlled by bit 6 (GP6) is unresponsive when driven low as per Microchips app notes and data sheets, while the other pins do respond. That was my question.

[PCM programmer]

1. Post a short, compilable test program that shows the problem.
2. Post your compiler version.
3. Post a description of the external circuits connected to the GP6 pin.