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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: Frustrated over converting 8 bit bcd to serial data
Date: Sat, 02 Nov 2002 18:40:50 GMT
NNTP-Posting-Date: Sat, 02 Nov 2002 10:40:50 PST
Randy Ott wrote:
> The Burr Brown part uses a three-wire SPI interface which requires more than
> a simple shift register.
> "Gary Tait" wrote in message
> > >You need a parallel to serial shiftregister.
> > >
> > Such as a 4021. Just run it twice.
In the case of the full SPI (4 wire), it really is almost as simple as a
shift register since enable can double as a strobe. It becomes a bit
gruesome when SDI and SDO are combined as one wire because the timing is
not well defined and some manufacturers do not present data quickly
enough for a single operation read. This is particularly true of some
modern devices with 128 registers and more. To make matters even more
interesting, there are several variants that use opposite polarity
enable, use inverted data or use the other clock edge to indicate data
valid. Other variants of SPI use configurable bit streams that can be of
any length (even Motorola has done this and Atmel has two chips in the
same family that use different polarity read/write bits). However, to
write to a peripheral only, the interface is quite simple. The SPI
horrors created by SSI and others are mainly problematic for reading
data from the peripheral.
BTW, one way around the readback problems on 3 wire SPI is to use a very
slow clock. This allows plenty of time for the data to ripple through
deeply nested multiplexers and load in the peripheral output shift
register. I have this working well with both SSI and Atmel parts.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons firstname.lastname@example.org
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