Post by thread:Q: I2C vs SPI

>I see many posts about I2C. Not so many SPI queries. >This leads me to suspect that I2C is more popular than SPI. >Why is this? > >Walt I suspect that there are fewer SPI posts because SPI is simpler. I'm not sure if it is less popular though. I personally prefer I2C EEPROMS, etc., just because you can save a pin or two and stick a bunch of them on the same bus if you want. With SPI you would at least have to connect seperate chip selects I think (is there any protocol for addressing an SPI device without using a seperate chip select?) On the other hand, SPI is generally quite a bit faster, especially if you can use dedicated hardware on the microcontroller (which is built in to many of the midrange or higher PICs). So I wouldn't be surprised if SPI is reasonably popular. Ken

> I see many posts about I2C. Not so many SPI queries. Because SPI is easier? > This leads me to suspect that I2C is more popular than SPI. > Why is this? Because I2C is better :) Specifically, the fact that I2C is a two wire bus with no Select lines needed means that for severely pin-limited applications (all of them!?) you can do more with less. Also because the most common application is external EEPROM, and MicroChip's EEPROMs are I2C interface. Because of that, there is code around for I2C on the PIC, so people tend to prefer it. Many Motorola microcontrollers have built in SPI support hardware, so in Motorola shops you see more SPI than I2C. ------------ Barry King, KA1NLH Engineering Manager NRG Systems "Measuring the Wind's Energy" Hinesburg, Vermont, USA http://www.nrgsystems.com

In allot of cases you can combine SO and SI and I hear tell that nCS can be made from SCK with a RC delay to get SPI down to 2 wires. James Newton spam_OUTjamesnewtonTakeThisOuTgeocities.com phone:1-619-652-0593 http://techref.homepage.com NOW OPEN (R/O) TO NON-MEMBERS! Members can add private/public comments/pages ($0 TANSTAAFL web hosting) {Original Message removed}

But unless there was some overwhelming reason to do so, wouldn't that be a lot of trouble when you could just use I2C to begin with? Just a thought.... Regards, Jim On Tue, 11 January 2000, James Newton wrote: > > In allot of cases you can combine SO and SI and I hear tell that nCS can be > made from SCK with a RC delay to get SPI down to 2 wires. > > James Newton .....jamesnewtonKILLspam@spam@geocities.com phone:1-619-652-0593 > http://techref.homepage.com NOW OPEN (R/O) TO NON-MEMBERS! > Members can add private/public comments/pages ($0 TANSTAAFL web hosting) > > {Original Message removed}

Quitt, Walter wrote: > I see many posts about I2C. Not so many SPI queries. > This leads me to suspect that I2C is more popular than SPI. > Why is this? Well, it may be because this is the PIC list, and not so many Motorola devices are discussed, SPI being a bit of a Motorola favourite, but it is probably *mostly* because IÓC is far simpler to implement in the hardware sense - only two wires. -- Cheers, Paul B.

For me it was having to use an Atmel 45d081 which only comes with a SPI interface and having 1.5 pins left in the design. I would pick IIC (which is right next to I2C <GRIN>) if I had the choice. James Newton (PICList Admin #3) jamesnewtonKILLspampiclist.com 1-619-652-0593 PIC/PICList FAQ: http://www.piclist.com {Original Message removed}

Yep, short SO and SI. Some parts (like the AT45d081) guarantee that they will not drive SI unless they are responding. nCS is my shorthand for Not Chip Select (since Chip Select is a negative logic signal) and I understand that it can be derived from an RC delay. You have to be very careful about how you drive SCK... keep it high until you need the chip, pull it low for a good long time then try to go high for as short a time as possible until done clocking data. Only one device, unless the devices listen for an address, that is correct. see http://204.210.50.240/techref/default.asp?url=atmel/at45d081.html James Newton .....jamesnewtonKILLspam.....geocities.com phone:1-619-652-0593 http://techref.homepage.com NOW OPEN (R/O) TO NON-MEMBERS! Members can add private/public comments/pages ($0 TANSTAAFL web hosting) {Original Message removed}

sweeping generalizations................... > Because I2C is better :) Not for a newbie... Specifically, the fact that I2C is a two wire > bus with no Select lines needed means that for severely pin-limited > applications (all of them!?) you can do more with less. You can do the SAME with LESS. And using the Select, CLK and SDI & SDO(tied together with a 1K resistor) you only lose 1 pin. > Also because the most common application is external EEPROM, I would think checking buttons would be the most common application... > and MicroChip's EEPROMs are I2C interface. They have a series of SPI-like and 'real' SPI EEPROMS. The 93 and 25(?) series > there is code around for I2C on the PIC, so people tend to prefer it. tony

> But unless there was some overwhelming reason to do so, wouldn't > that be a lot of trouble when you could just use I2C to begin > with? Just a thought.... > > > > In allot of cases you can combine SO and SI and I hear tell that nCS can be > > made from SCK with a RC delay to get SPI down to 2 wires. ^^^^^^^^^^^^^^^^^^^^^^ This is trouble? tony

> > I see many posts about I2C. Not so many SPI queries. > > Because SPI is easier? > > > This leads me to suspect that I2C is more popular than SPI. > > Why is this? > > Because I2C is better :) Specifically, the fact that I2C is a two > wire bus with no Select lines needed means that for severely > pin-limited applications (all of them!?) you can do more with less. > So is DALLAS's 1-wire better yet? ;)

>So is DALLAS's 1-wire better yet? ;) An EEPROM on this interface would sure be handy! Especially for some 12Cxx projects (and especially if you are using DS1820 temperature sensors, etc., anyway .. just drop an EÓ on the same line!). Is there any EÓ available on this interface? Do they have 1M or 4M in SOT-23s? :o) Ken

Not too sure, but maybe it's because there are lot more various I2C chips, such as : - adc<->dac - tuners - lcd controllers - eeprom - ram - audio - video -... Just my 2... "Quitt, Walter" a Žcrit : > > I see many posts about I2C. Not so many SPI queries. > This leads me to suspect that I2C is more popular than SPI. > Why is this? > > Walt -- --------------------------------------------------------------------- Philippe Jadin EraseMEphilippe.jadinspam_OUTTakeThisOuTskynet.be Belgium, Europe Simple yet usefull Robot stuff goto http://users.swing.be/philippe.jadin For low-cost web design goto http://users.skynet.be/clairetnet