topic Re: CT_Bias Timing in 8-bit Microcontrollers

CT_Bias Timing

DedsBee — Mon, 24 Nov 2008 23:42:09 GMT

Hi all,

Is there any way to adjust the time between the change of state of CT_Bias pin (in MC13213) and the start of transmission/reception?

I´m using Beestack and I´m almost sure that this is impossible to do via software. And I can´t find any register to do that. Anyone knows?

If the time of switching CT_Bias is fixed, I think that there is some time to warmup and some time to warmdown (after transmission), am I right? Anyone knows exactly the time of it? I need that to overcome possible timing issues with an external PA/LNA. Thank you all!

Regards,

DedsBee

Re: CT_Bias Timing

RichTestardi — Tue, 25 Nov 2008 07:21:44 GMT

I do not know of a way to change the timing of CT_BIAS.

> I think that there is some time to warmup and some time to warmdown (after transmission), am I right?

I measured some of this for CT_BIAS here:

http://forums.freescale.com/freescale/board/message?board.id=8BITCOMM&message.id=12549#M12549

That seemed to confirm what was in MC13202RM.pdf in section 3.10.3.1.3:

3.10.3.1.3 Modem TX Timing profile

The transmit or TX timing profile is more predictable than the RX profile. Figure 3-10 shows the timing profile for a TX operation. There is the usual initial 144 μs warm-up period from idle current to full TX current (typically 30 mA) followed by the TX operation (TX is covered in detail in Section 6.3, “Active Modes”). The TX time is not a set figure but it is predictable.

The raw transmission rate of the IEEE 802.15.4 2.4 GHz physical layer is 62.5 ksymbols/s or 250 kb/s. This means the TX time for 2 symbols or 1 byte of data is 32 μs. An IEEE 802.15.4 2.4 compliant packet has 4 bytes of preamble, 1 byte of SFD, 1 byte of FLI, 2 bytes of FCS plus the payload data (125 bytes maximum). As a result the, the overhead of a frame is 8 bytes or 8 x 32 = 256 μs, and the maximum payload TX time is 125 x 32 = 4000 μs. The TX time for a packet then is:

Total TX time (μs) = 256 + (payload bytes x 32)

The TX operation will end after the FCS bytes are sent. The return to Idle has a “warm down” period of 10 μs to allow the RF transmitter to taper off in a manner to avoid RF “splatter”.

Re: CT_Bias Timing

DedsBee — Tue, 25 Nov 2008 08:40:25 GMT

Thank you Rich! That´s exactly what I need. 144 us warm-up period is good enough for me to enable the external PA.

Thanks again!

Regards,

DedsBee