User manual

7. Advanced SNAPpy Topics

This secon describes how to use some of the more advanced features of SNAP. Topics covered include:

• Interfacing to external CBUS slave devices (emulang a CBUS master)

• Interfacing to external SPI slave devices (emulang a SPI master)

• Interfacing to external I

C slave devices (emulang a I2C master)

• Interfacing to mul-drop RS-485 devices

• Encrypon 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 ming 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 exisng readPin() and writePin() funcons, 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() funcon. The CLK, CDATA, and CS pins would be controlled using the writePin()

funcon. The RDATA pin would be read using the readPin() funcon.

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

with strict ming requirements. Opmized nave code may be preferred over the SNAPpy virtual machine in

such cases.

To solve this problem, dedicated CBUS support (master emulaon only) has been added to the set of SNAPpy

built-in funcons. Two funcons (callable from SNAPpy but implemented in opmized C code) support reading

and wring CBUS data:

•

cbusRd(numToRead) – “shis in” the speciﬁed number of bytes

•

cbusWr(str) – “shis out” the bytes speciﬁed by str

To allow the cbusRd() and cbusWr() funcons 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

For plaorms other than the RF100, refer to the appropriate plaorm speciﬁc secon in the back of the SNAP

Reference Manual.

NOTE– These pins are only dedicated if you are actually using the CBUS funcons. If not, they remain

available for other funcons.

You will also need as many Chip Select pins as you have external CBUS devices. You can choose any available

GPIO pin(s) to be your CBUS chip selects. The basic program ﬂow becomes:

# select the desired CBUS device

writePin(somePin, False) # assuming the chip select is active-low

38 SNAP® Network Operang System