CC2420 Power Consumption Measurements on the SPOTs
By user12611170 on May 15, 2009
On the SunSPOT world forums, there was a query regarding the power consumed by the TI CC2420 radio transceiver. The SPOT has quite a bit going on and the power consumption is very dynamic. The only way I could see to get an accurate reading was to isolate the power into the radio chip. There is a ferrite bead, FB1, in series between the switcher and the radio VDD that could be removed and a milliammeter put in its place.
We found a working rev 6 eSPOT (s/n 2DA1) and loaded up red-090429 release. Del removed the bottom shield and ferrite bead and connected the test wires. We used a calibrated Agilent 34410A digital multimeter for the readings. This is a free range SPOT with eDEMO board and battery. It was battery powered during the test and the battery was fully charged.
I used a program written by Syntropy for our FCC compliance test. This test can broadcast both modulated direct sequence spread spectrum and unmodulated carrier on channels 11, 18 and 26. The power output of the transmitter can be adjusted between 0dBm to -24dBm. The test receives on channels 11, 18 and 26 and will indicates packet errors in the LEDs.
The CC2420 is a transceiver - either the transmitter is on or the receiver is on but you cannot have both on at the same time. When the SPOT is active (not in deep sleep) the CC2420 draws 0.44ma without either transmitter or receiver enabled.
The receiver drew 18.57ma (ch11), 18.49ma (ch18) and 18.61ma (ch26) without a nearby transmission. The receive current drops about 30uA when receiving packets from a nearby transmitter. It did not vary noticeably if the packets were sent at low or high signal levels.
Transmit power drew at full output power modulated 18.24ma (ch11), 18.18ma (ch18) and 18.05ma (ch26). At full power unmodulated carrier, it consumed 18.04ma (ch11), 17.98ma (ch18) and 15.54ma (ch26). The higher power setting of ch26 is forced to a lower power output to be compliant with FCC emission requirements for the ISM band. The following is a graph of the current versus RF output power. There was very little variation switching between channels.
The measured transmit current of 18.24ma was higher by 0.82ma than the typical specification rating of 17.4ma. The measured receive current of 18.57ma was lower by 1.13ma than the specified typical of 19.7ma. The idle mode was slightly higher by 14uA with 440ua measured and 426uA specified.
There were minor rapid fluctuations of power during the test. The transmit power would fluctuate about 20-30uA and the receive current fluctuated <7uA (not including the variation caused by nearby transmitter). The values measured were average values over these fluctuations.
If there are other power measurements on the radio that need to be done, let me know. I'll keep this modified SPOT around for that.