2.4 ghz WiFi ID

[Christopher4187]

Thanks for the reply. I'm just entering the world of wifi with my projects so I have a few more questions.

Am I not reading the datasheet correctly or does the RN-171 not have SPI? If not, can I use something other than a USART to communicate with it? The reason I ask is because I'm using the USART for another chip. I don't know if it's like SPI where the USART pins go to high impedance mode when the CS line is deasserted.

I can't code in C but is it needed for the MRF24WB0MB?

Regarding the TCP/IP stack, both datasheets state they include one. Why is it so much harder to use the Microchip MRF24WB0MB as opposed to the RN-171?

For both models, can you use them to communicate with a identical model (peer to peer) and also through the internet?

[HenrikOlsson]

Hi Christopher,
Looking at the datasheet for the MRF24WB0MB it's my understanding that it does NOT contain the TCP/IP stack. On page 7 it says

The MRF24WB0MA/MRF24WB0MB modules are designed to be used with Microchip’s TCP/IP software stack.

The combination of the module and a PIC running the TCP/IP stack results in support for IEEE Standard802.11 and IP services.

So if I don't read that completely wrong you need to run a TCP/IP stack in the PIC you're using to drive the MRF24WB0MB. Alternatively you can pair it up with the MCW1001A which I suspect is just a PIC preprogrammed with Microchips TCP/IP stack and some user interface code. What you'll end up with would then be something like this.

You don't NEED to code in C to use MRF24WB0MB directly with a PIC but the free Microchip TCP/IP stack is written in C and to use THAT you'll need to write your application in C. Obviously you COULD develop your own TCP/IP stack in PBP but that is a monumental task and then some. Fred Eady (I think) has posted some TCP/IP code for PBP on this forum in the past but I have no idea what's implemented and in what state it is.

The RN-171 on the other hand does seem to have the TCP/IP stack already implemented within the module and if you look at the features you'll see

•Hardware interfaces: UART and SPI Slave

And in the datasheet it says

• Host data rates up to 921 Kbps TX, 500 Kbps RX for the UART, up to 2 Mbps over the SPI slave

Please note that I don't have any experience with either module. I do have a MRF24WB0MB + MCW1001A module but haven't done anything with it.

/Henrik.

[Christopher4187]

Henrik,

Thanks for the information. I'm not up to the task of writing any coding for TCP/IP since I know almost nothing about it. Regarding the MRF24WB0MB, what I meant was that Microchip provides it so it's "there." However, if it requires me to modify it with C, that option is out for me.

Regarding the RN-171, the reason why I asked about SPI is due to the fact that the sheets 4, 5 and 6 in the datasheet all state NOT to connect the SPI interface, which is on pins 15, 16, 17 and 18. Also, I thought that SPI is quicker than using the UART, no?

[HenrikOlsson]

You're right, that IS confusing.
It says right there that it supports SPI slave but then in the table over the pins it says Unused/Do not connect on the SPI pins. I Googled real quick and it seems like we're not the only ones that are confused..... If you find an answer please let us know!

SPI can be (and generally is) conciderably faster then asynchronous (UART) but it depends on what the master AND slave devices are capable of. For example, if you set the USART up for 250kbit that will be conciderably faster then using bit-banged SPI (SHIFTOUT/SHIFTIN) but slower than using the MSSP module to do "hardware SPI" - which of course is the recommended way.

Using the MRF24WB0MB + MCW1001A combo doesn't allow SPI between the PIC and network module either - it's asynchronous (UART) only I'm afraid.

/Henrik.

[Christopher4187]

Ok, so here's a question I can't seem to find an answer to. I'm currently using RF modules that use the USART on my 18F4550 (Pins 1 and 44). Is the USART similar to the SPI where you can attach a number of devices and have the data lines go to high impedance mode when the CS line is driven high?

My ultimate problem is that the RF devices use the USART, the MCP2515 uses the SDO pin and the RN-171 uses the USART (although I assume based upon what you are saying I may be able to use some other pins but the speed would be slower).

In re-reading the datasheet, do you think this line:

• Host data rates up to 921 Kbps TX, 500 Kbps RX for the UART, up to 2 Mbps over the SPI slave

means that using the UART is faster than software SPI?

[HenrikOlsson]

No, not in the way that SPI works. However with a bit of hardware you can do it. For example you can use a CD4066 to switch between devices. Another option is to use a PIC with two usarts. It all depends....

Perhaps you can use the USART for the RN-171, use SHIFTIN/SHIFTOUT (or the MSSP module) for the MCP2515 and use SERIN/SEROUT for the RF-module. It all depends....

The speed of the bit-banged SPI (ie SHIFTOUT/SHIFTON) commands depends on the osciallator. The manual says 50kHz dependant on oscillator (I suspect 4MHz) and that the active state is held to a minimum of 2us. I interpret that as no matter how fast the PIC runs the active state will always be 2us, assuming that the clock is symetric that means 4us per bit or max 250kHz. I think that in reallity it'll be quite a bit less than that. So, if you're comparing the USART to bit-banged SPI I'd say it's likely that the using the USART will be faster and because receving and transmitting is handled by hardware the PIC can do other things - if used properly.

SHIFTIN/SHIFTOUT can be used on any pins of the PIC - not just the SDI/SDO pins.
Using the MSSP module to do "hardware SPI" requires the use of the SDI/SDO pins since that is the pins to which the MSSP module is connected. SHIFTIN/SHIFTOUT does not use the MSSP module.
SERIN/SEROUT can be used on any pins of the PIC - not just the TX/RX pins.
Using the USART to do "hardware asynchronous" communications requires the use of the TX/RX pins since that is the pins to which the USART module is connected. SERIN/SEROUT does not use the USART. HSERIN/HSEROUT does.

[Christopher4187]

I've given this some thought. I think I'm going to get rid of the RF devices and just use the MCP2515 on the SPI (I want to keep this because I can add other SPI devices with ease) so that leaves where to put the RN-171. Isn't the MSSP on the same pins as the SPI? If so, what about using the SC16IS750 for the RN-171? I've never used it but it looks like it should work. What do you think?