Servo - November 2011

PHOTO 6. That SP233ACP works just like the legacy MAX232 but sans charge pump capacitors. This lashup is perfect for adding RF capability to pre-USB equipment.

2,400 bps, 4,800 bps, 9,600 bps, 19,200 bps and 38,400 bps. The channels and baudrates are software selectable.

The wireless transceiver module is designed to communicate with a host microcontroller’s UART using TTL voltage levels. VDD, GND, TX, and RX are the only physical connections the SRWF-2501 module accepts.

On-module intelligence for the SRWF-2500 is provided by an Atmel ATMega88. The ATMega88 is a general-purpose microcontroller endowed with 8 KB of Flash and 1 KB of SRAM. The ATMega88’s Flash contains the Sunray Technology version of a CC2500 driver, as well as the baud and channel configuration firmware. The Sunray Technology CC2500 driver is packet oriented and can only belt out a full CC2500 buffer per transmission. So, the Sunray module is perfect for small packet low rate data transfer applications.

It’s a no brainer to deduce that a copilot microcontroller can be attached to the transceiver’s TTL-compatible serial interface. The ATMega88’s firmware provides an aim and shoot platform. That is, every character that enters the module’s TX input is transmitted without any intervention by the host. Likewise, any data received by the SRWF-2501 module’s CC2500 is automatically presented to the RX output.

In that the Sunray Technology module is “smart,” we can deploy the SRWF-2501 in a number of interesting ways.

For example, Photo 4 captures an SRWF-2501 module tied to a Microchip MCP2200 USB 2.0 to UART protocol converter with GPIO.

This particular configuration allows the Sunray Technology module to attach to a PC’s USB portal.

When using the SRWF-2501 wireless transceiver module with a Look up “handy” in the dictionary and you’ll find that its definition is Digilent Pmods. If you’ve worked with the Digilent Cerebot 32MX7, you may have a PmodUSBUART lying around on your bench. The PmodUSBUART is based on the FTDI FT232RQ USB-to-UART bridge and is the replacement for the PmodRS232. The local (LCL) and system (SYS) jumper points in Photo 5 allow you to control the power associated with the PmodUSBUART. Basically, if the equipment that is attached to the PmodUSBUART is powered on its own, the jumper should be set to LCL. Otherwise, jumpering to SYS allows the PmodUSBUART to supply + 3. 3 volts to the attached equipment on its SYS3V3 pin. You can get all of the scoop on the PmodUSBUART from the Digilent website.

It took quite a while for me to embrace USB. There may be some of you out there that are still on the fence.

Or, maybe your piece of robotic equipment does not have USB capability. You can wire up a MAX232 and a sextet of charge pump/bypass capacitors. Or, you can drop a Sipex SP233ACP in its place. The SP233ACP doesn’t require any charge pump capacitors. My SP233ACP-equipped SRWF-2501 wireless transceiver module is the subject of Photo 6.