Comments on: Using SPI on an AVR (1)

By: andrew

andrew — Thu, 08 Jul 2010 17:04:39 +0000

Atmel datasheet states that max SPI rate is clock/2 in master mode and clock/4 in slave. So in theory yes if run at 16MHz. But do any of the ATtiny chips have hardware SPI built in?

By: udi

udi — Thu, 08 Jul 2010 13:42:47 +0000

Hi All,
Can anyone state what is the fastest SPI clock you’ve reached with ATtiny?
I need a super low power solution with 8MHz SPI. is this doable ?
uDi

By: andrew

andrew — Sun, 16 May 2010 08:46:40 +0000

ahh yes, typo – I’ll fix in the page. It wouldn’t make any difference in the example above but would if multiple devices were on the same SPI bus.

By: Sony

Sony — Sun, 16 May 2010 08:14:03 +0000

I know that but turn on MCP3201 twice. In main before you setup SPI and again
when you read_adc. Confusing.
Moreover in that code different. Why?
http://code.google.com/p/rocketnumbernine/source/browse/trunk/AVR/spi/Arduino/SPI/examples/MCP3201_ADC/MCP3201_ADC.pde

By: andrew

andrew — Sat, 15 May 2010 20:14:56 +0000

The MCP3201 requires a low value on its chip select pin to select the device. SELECT_ADC sets the pin (connected to PB4) to 0:
#define SELECT_ADC PORTB &= ~(1<

Hope that makes sense.
Andrew

By: Sony

Sony — Sat, 15 May 2010 18:27:06 +0000

Hello,

”
// make sure ADC is unselected and setup spi
SELECT_ADC; ”

Why do you have “SELECT_ADC”?
I think that “DESELCT_ADC” is correct?
If i am wrong explain me it please.
Regards,
Sony

By: andrew

andrew — Mon, 03 May 2010 15:14:06 +0000

The left shift operator shifts the entire bit pattern on the left of the operand (0×1F & one) by the number on the right (7), filling with zeros. As were shifting into a 2 byte variable we’ll end up with 0000A1A2A3A4 A50000000.
regards
Andrew

By: Sony

Sony — Mon, 03 May 2010 10:01:18 +0000

Thanks for the quick replay but i have another question:

“((0×1F & one) << 7) "

for example as you mention we have:

(0×1F & one) = 000A1A2A3A4A5 (A1..A5 single bit).
When you shift left everything 7 bit you get this:

A5000A1A2A3A4?

By: andrew

andrew — Mon, 03 May 2010 07:46:55 +0000

You’re right the _delays are not required – they were just added to simplify observation with a logic analyzer, the example Arduino Sketch that’s shiped with SPI library doesn’t include them – see here.

By: Sony

Sony — Sun, 02 May 2010 21:42:38 +0000

Hello,

i have 3 question about time delay in your program.

1) in function read_adc()
after you power on adc you have:
_delay_us(100)
why so long? 3) in datasheet CS disable time is 625 ns so 1us delay wiil be enough?

2)
” unsigned char one = send_spi(0xFF);
_delay_us(100);”

why you delay after you send first byte?;>

” PORTD = (1<> 9)); // use the top 3 bytes to turn on LED
_delay_ms(1);”

_delay_ms(1) – what for is that delay?

My English is not very good but i hope that you understand me.

Regards,
Sony