topic Re: SLew rate Clock in LPC Microcontrollers

SLew rate Clock

lpcware — Wed, 15 Jun 2016 18:27:06 GMT

Content originally posted in LPCWare by namorada on Tue Jun 23 00:03:23 MST 2015
Hi,

I'm trying to make an SPI connection with a slave. The master is, of course, the LPC18xx.
The SCK signal is populated on P1.19 (also tried on CLK0, but same result).
The PLL1 is configurated for 180MHz and clock the SSP1 module (used on SPI).

I try to make a connection to 15MHz SPI. When I look at the clock signal SCK, it is so rounded that it can work. After brief tests, it turns out I can not achieve SPI communication over 6Mhz.
I have initialized the pin like that :

LPC_SCU->SFSCLK[0] = (u32) (SCU_MODE_HIGHSPEEDSLEW_EN | SCU_MODE_FUNC6);

Where :

#define SCU_MODE_HIGHSPEEDSLEW_EN  (0x1 << 5)/*!< Enable high-speed slew */
#define SCU_MODE_FUNC6             0x6/*!< Selects pin function 6 */

It is weird that the LPC18xx can get more 6Mhz but are not really too rounded ... Some among you have already encountered this problem ?

Thank you,
David.

Re: SLew rate Clock

lpcware — Wed, 15 Jun 2016 18:27:07 GMT

Content originally posted in LPCWare by namorada on Wed Jul 01 00:05:25 MST 2015
Hello!
No one, no Idea? :-(

Re: SLew rate Clock

lpcware — Wed, 15 Jun 2016 18:27:08 GMT

Content originally posted in LPCWare by starblue on Wed Jul 01 01:15:31 MST 2015
I would guess it is a problem with your layout.
Do you have long wires or a big loop between SCK and ground?

Re: SLew rate Clock

lpcware — Wed, 15 Jun 2016 18:27:09 GMT

Content originally posted in LPCWare by namorada on Wed Jul 01 02:31:48 MST 2015
Nope.
The signal you can see in my first post was taken with physical disconnection between the pin and the solder pad.

Re: SLew rate Clock

lpcware — Wed, 15 Jun 2016 18:27:09 GMT

Content originally posted in LPCWare by bavarian on Fri Jul 10 02:40:57 MST 2015
If you make sure that the correct pin configuration registers for the SPI interface pins gets initialized to use

[list]
[*] fast slew rate
[*] no internal pullup/down
[*] input buffer enable
[*] no glitch filter
[/list]

then you should have a perfect square wave signal at this frequency.

Regards,
NXP Technical Support Team