AM3358: Setting SPI bus rate

Jeff cohen

Part Number: AM3358
Other Parts Discussed in Thread: MSP430F149, TLV5616

Dear TI Support,

I'm designing an industrial control system based on the Beaglebone Black Rev C that has a 4-20mA current loop. One SPI Bus will be dedicated to the 4-20mA loop transmitter and the other will be dedicated to the 4-20mA input.

I see that the SPI Bus Clocks are all integer dividers of 48MHz.

On the transmit side, this makes it impossible to generate the SPI bus clock frequency that would work for many of the standard serial transfer rates I.e. 19.2K for a 16-bit transfer would require a baud rate of 3.072MHz, which is not an integer quotient of 48MHz. The closest data rate I can get to is 3MHZ, which makes for a data rate of 18.75kbs, which is too far away from 19.2Kbps Error = (18.75/19.2) -1 = 2.3% I need less than 2%

Is there a way to change the 48MHz clock that drives the SPI Bus?

Thank you very much for all of your help and support.

Jeff Cohen

Hytorc

over 8 years ago

0 Biser Gatchev-XID over 8 years ago

TI__Guru**** 393215 points

Hi,

The McSPI functional clock is taken from the Peripheral PLL M2 output (192MHz) and then divided by 4 (48MHz). To change this frequency you have to change the Peripheral PLL frequency, and this will affect the all subsystems that are clocked by it (practically all device peripherals). In short, it's strongly recommended not to try changing this frequency.

I don't understand why you are worrying about the 19.2K (by the way is this kilobits or kilosamples per second)? In your case the AM335x is the SPI Master and the Slaves will work at whatever SPI clock they are fed, as long as it's within their operating limits.

0 Jeff cohen over 8 years ago in reply to Biser Gatchev-XID

Prodigy 80 points

Hi,

Thank you very much. I agree that it makes sense not to change the value of the PLL M2 frequency of 192MHz.

Please allow me to explain that there is a discrepancy between the desired SPI bus clock frequency and the actual attained frequency.On the transmit side I am okay up to 230.4kbs. Most receivers specify a 2% error rate. The table below shows the data rate, the desired SPI Clock frequency and the error rate. Note that Error rate =[ 1 - (actual/desired)]X 100].

Data Rate	Desired SPI Bus Clock *16 Frequency	Divisor Value	Actual SPI Bus Clock Frequency	Actual Data Rate	Error %
2400	38400	1250	38400	2400	0
4800	76800	625	76800	4800	0
9600	153600	312.5	153846.15	9615.4	0.16
14400	230400	208.33	23079	14423	0.16
19200	307200	156.25	307692	19231	0.16
38400	614400	78.125	615385	38462	0.16
57600	921600	52.083	923077	57692	0.16
115200	1843200	26.0416	1846154	115385	0.16
230400	3686400	13.0283	3692308	230769	0.16
460800	7372800	6.51	6857143	428571	-7.0
921600	14745600	3.255	16000000	1000000	8.6

Regards,

Jeff

Hytorc Inc. Mawah, NJ USA

0 Biser Gatchev-XID over 8 years ago in reply to Jeff cohen

TI__Guru**** 393215 points

Do you mean that the SPI slaves (4-20mA peripherals) require a continuous SPI clock? Can you specify the part numbers, or post datasheets?

0 Jeff cohen over 8 years ago in reply to Biser Gatchev-XID

Prodigy 80 points

Hi,

The clock is not continuous. IT is only active during transmission of data over the 4-20mA loop. Typically this type of communications is handled by an MSP430 micro-controller.

The system looks similar to that which is shown in Figure 1 of "Using SPI Synchronous Communications With Data Converters: Interfacing The MSP430F149 and TLV5616"
Link: www.ti.com/.../slyt137.pdf

The SPI bus is usually idle, and whenever there is a packet of serial data, the data is transferred in a burst, and on every 16th data transfer, the data is loaded into the DAC and converted. Can you think of a better approach?

Regards,

Jeff
Jeff Cohen: Hytorc INc, Mawah, NJ USA

0 Biser Gatchev-XID over 8 years ago in reply to Jeff cohen

TI__Guru**** 393215 points

What I don't understand is why the data should be transferred at exactly these baudrates. The table above is true for UART interfaces, which are asynchronous, but SPI data is clocked in the slave device by the master clock.

0 Jeff cohen over 8 years ago in reply to Biser Gatchev-XID

Prodigy 80 points

Hi,

You are right. The baud rate does not matter. These sensors are not intelligent devices. They are analog transducers.

I was under teh impression that they can carry data too. I was mistaken.

Thank you very much for pointing this out to me.

Regards,

Jeff

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AM3358: Setting SPI bus rate