SPI communication between PIC18F4550 and ADS1254 ADC

JosedeJesusC · Joined: 29 Mar 2013 Posts: 24

Hi!!!! everyone

I'm trying make a SPI communication between PIC 18F4550 and ADS1254 (Delta sigma ADC) but the adc sends a 24 bits word into a 1 bit stream.
My trouble is:

1. How can I receive the data if the function SPI_READ() just give me 8 bits ?
2Reading the compiler manual, it makes mention that by using #USE SPI, I can modify the word size to be sent and specify the name of the variable where is stored this word.
Is not it right?

2. The ADS1254 indicate across BIT SDO which the data is ready. and then the data is shifted out in the same pin, I suppose the #INT_SSP and read_spi() give it to me the result or I should wait some time to receive data.
Like you see, I have many questions.
I hope that someone can help me smile.
best regard!!!!!! Very Happy

alan · Joined: 12 Nov 2012 Posts: 350 Location: South Africa

The way I do it, maybe there are better ways:

JosedeJesusC · Joined: 29 Mar 2013 Posts: 24

Hi Alan thanks for your reply !!

OK! like I said there are options to drive the amount of bits you want to send but I'm not sure.
I'm going to use your example code, if the way using the preprossesing directive #USE SPI is right I am in glad to tell you.

best regards!!!

alan · Joined: 12 Nov 2012 Posts: 350 Location: South Africa

Yes that will be when you use software SPI and write your own bit-banging functions.

Regards

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

asmboy · Joined: 20 Nov 2007 Posts: 2128 Location: albany ny

I have used this part , and it helps to understand HOW to use it.

Do you change the input MUX settings or stay on a single port ?

There are ways to get into trouble with the ADS1254 in terms failure to
understand the Dout/Dready function.

Your code makes no provision for knowing WHEN to begin clocking out the data you want. Otherwise you risk getting junk results

the datasheet page 11,12,13 are essential reading if you want your driver to actually work.

perhaps you'd like to post more code ?

JosedeJesusC · Joined: 29 Mar 2013 Posts: 24

Hi asmboy and PCM Programmer.
Thanks so much both for you reply!!!!

I have some doubts.
The master must wait 36 MCLK cycles till data is ready to read like the data sheet says because the pin SDO/DRDY has double functions as a flag to indicate the data is ready and as a port to send the 1bit stream that represents the result.
The function spi_read(0) or #INT_SSP makes all that by default ------ or I must get in a delay before to read Confused

what does mean that? SPI_H_TO_L | SPI_XMIT_L_TO_H .

About with SPI communication.... if the master works with 48 MHz and SPI prescaler of 16,
Then the frequency of SPI should be 3 MHz. is it right?.

Respect with the MUX. I work with one channel. But after to get the driver of the DAS1254, I'm going to extend to 4 channels. Like you said asmboy I will get in trouble if the master makes not following steps:
Do not change from channel till the data is ready.
So first read the data and then change from a channel to other.
Hide first 5 results because this ADC works averaging old data.

I'll post the code please wait me a time.

asmboy · Joined: 20 Nov 2007 Posts: 2128 Location: albany ny

the code i wrote to support this part uses bit banging NOT the spi interface at all.
That made it much simpler to manage the data stream, at least for my use of the part.

LOOK at the timing diagrams and keep your eye on SCLK after last bit of data is clocked out..

it is just not very suited to simple SPI transfer , IMHO-

let me add that Dout is used as a combined "ready flag" and data_out pin on MANY of the BB/TI low end A/D chips, and once you master this one, many others become EZ to control as well.