Specifications

6. Advanced SNAPpy Topics

This section describes how to use some of the more advanced features of SNAP. Topics covered

include:

• Interfacing to external CBUS slave devices (emulating a CBUS master)

• Interfacing to external SPI slave devices (emulating a SPI master)

• Interfacing to external I2C slave devices (emulating a I2C master)

• Interfacing to multi-drop RS-485 devices

• Encryption between SNAP nodes

• Recovering an unresponsive node

Interfacing to external CBUS slave devices

CBUS is a clocked serial bus, similar to SPI. It requires at least four pins:

• CLK – master timing reference for all CBUS transfers

• CDATA – data from the CBUS master to the CBUS slave

• RDATA – data from the CBUS slave to the CBUS master

• CS – At least one Chip Select (CS)

Using the existing readPin() and writePin() functions, virtually any type of device can be interacted

with via a SNAPpy script, including external CBUS slaves. Arbitrarily chosen GPIO pins could be

configured as inputs or outputs by using the setPinDir() function. The CLK, CDATA, and CS pins

would be controlled using the writePin() function. The RDATA pin would be read using the readPin()

function.

The problem with a strictly SNAPpy based approach is speed – CBUS devices tend to be things like

voice chips, with strict timing requirements. Optimized native code may be preferred over the

SNAPpy virtual machine in such cases.

To solve this problem, dedicated CBUS support (master emulation only) has been added to the set of

SNAPpy built-in functions. Two functions (callable from SNAPpy but implemented in optimized C

code) support reading and writing CBUS data:

• cbusRd(numToRead) – “shifts in” the specified number of bytes

• cbusWr(str) – “shifts out” the bytes specified by str

To allow the cbusRd() and cbusWr() functions to be as fast as possible, the IO pins used for CBUS

CLK, CDATA, and RDATA are fixed. On an RF100 SNAP Engine:

• GPIO 12 is always used as the CBUS CDATA pin

• GPIO 13 is always used as the CBUS CLK pin

• GPIO 14 is always used as the CBUS RDATA pin

Page 34 of 202 SNAP Reference Manual Document Number 600-0007K