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Keywords:
SPI,electricity meter,microcontroller,communication
APPLICATION NOTE 4246
How to Use Serial Peripheral Interface (SPI) on
the MAXQ3180 Microcontroller
By: Ben Smith
Jun 17, 2008
Abstract: The MAXQ3180 microcontroller is a polyphase analog front-end for an electricity meter. It
incorporates all functions needed for modern, multifunction electricity metering. The MAXQ3180
communicates its readings to a host microcontroller by means of a Serial Peripheral Interface (SPI™)
bus. This application note describes how this interface is done, and presents sample code to assist the
designer in implementing the communications mechanism.
Overview of SPI
The Serial Peripheral Interface (SPI) is an interdevice bus protocol that provides fast, synchronous, full-
duplex communications between chips. One device, the
master,
drives the synchronous clock and
selects which of several
slaves
is being addressed. Every SPI peripheral consists of a single shift register
and control circuitry so that an addressed serial peripheral interface SPI peripheral is simultaneously
transmitting and receiving.
Figure 1. Illustration for a SPI slave.
There are four discrete circuits used in SPI communication:
SCLK:
The synchronous clock used by all devices. The master drives this clock and the slaves
receive the clock. Note that SCLK can be gated and need not be driven between SPI transactions.
MOSI:
Master out, slave in. This is the main data line driven by the master to all slaves on the SPI
bus. Only the selected slave clocks data from MOSI.
MISO:
Master in, slave out. This is the main data line driven by the selected slave to the master.
Only the selected slave may drive this circuit. In fact, it is the only circuit in the SPI bus
arrangement that a slave is
ever
permitted to drive.
SSEL:
This signal is unique to each slave. When active (generally low) the selected slave must
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drive MISO.
For this discussion, it is critical to note that
a SPI peripheral transmits and receives at the same time.
A
handy way to think of this is that the master always sends one byte and receives one byte.
Some SPI peripherals sacrifice speed in favor of simulating a half-duplex operation. This is not the case
with the
MAXQ3180
microcontroller, which is truly a full-duplex SPI slave.
The remainder of this application note describes how to connect and successfully use the MAXQ3180 on
a SPI bus.
MAXQ3180 Communications Overview
To the master (i.e., host), the MAXQ3180 looks like a memory array that consists of both RAM and
ROM. This is because the ROM firmware in the MAXQ3180 reads its operational parameters from RAM,
and places its results in RAM. Consequently, configuring a MAXQ3180 is as simple as performing a
block write to its RAM locations.
Some MAXQ3180 "memory" locations trigger actions within the device to calculate electricity metering
results "on the fly." Writes to these locations are a "nop." The specific function and purpose of RAM and
virtual ROM locations are beyond the scope of this document. The important fact here is that the
microcontroller really has only two operations for SPI communication: read and write.
Every transaction in the MAXQ3180 begins with the master sending two bytes that contain the command
(i.e., read or write), the address to access, and the number of bytes to access. As mentioned above,
every SPI peripheral returns a byte for every byte received. The MAXQ3180, therefore, returns 0xC1
after receiving the first command byte, and 0xC2 after the second command byte. The protocol is
illustrated below in
Figure 2.
Figure 2. Master reads and writes data to the MAXQ3180.
If the master is reading one or more bytes, it must send dummy bytes. Remember that the master
cannot receive
anything
from the slave unless it sends something: send a byte to get a byte. But after
receiving a command, the MAXQ3180 may have to calculate the result and thus may not have it ready
when the master sends the dummy byte. For this reason, the MAXQ3180
always
sends zero or more
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bytes of a NAK character (0x4E or ASCII 'N') followed by an ACK character (0x41, or ASCII 'A') before
sending the data.
If the master is writing one or more bytes, it sends the data to be written
immediately
after sending the
command. The MAXQ3180 returns ACK (0x41) for each data byte. It then returns NAK (0x4E) until the
write cycle is complete, after which it returns a final ACK.
Note that immediately after the final ACK, the MAXQ3180 is ready to begin the next transaction; it does
not need to wait for any other event. It is not even necessary to toggle SSEL to begin the next
transaction. The MAXQ3180 knows that the first transaction is over and is ready for the next.
If, for whatever reason, it is necessary to reset the communications between the master and the
MAXQ3180 (for example, if communication becomes unsynchronized) the master need only wait 200ms
before restarting communication from the first command byte. A 200ms delay informs the MAXQ3180
that the master is abandoning the previous transaction.
Command Bytes
The
1.
2.
3.
command bytes tell the MAXQ3180:
Whether the requested transaction is a READ or WRITE
The length of the transaction
The address in RAM to modify (or the virtual ROM address to read)
Figure 3. Structure of the command bytes.
The first command byte (Figure
3)
tells the MAXQ3180 whether the transaction being requested is a
READ or a WRITE, and the length of the transaction. The command byte uses the following schedule:
Length Code Data Length
0b00
0b01
0b10
0b11
1 byte
2 bytes
4 bytes
8 bytes
The remainder of command byte 1 and all of command byte 2 provide the address of the byte in RAM
(or the identity of the virtual ROM function) to be accessed.
Host Software Design
Although the MAXQ3180 contains a hardware SPI controller, individual message bytes are still
processed in a software routine contained in the ROM firmware. For this reason, a delay is necessary
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between successive bytes. In current versions of the MAXQ3180, this delay must be no less than 100µs
for reliable operation. See
Figures 4
and
5.
Figure 4. Flowchart for reading from the MAXQ3180.
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Figure 5. Flowchart for writing the MAXQ3180.
Code Listings
Code is provided for interfacing a
MAXQ2000
microcontroller with a built-in SPI master to the
MAXQ3180. Users of other microcontrollers will need to provide their own SPI primitives and possibly
modify the high-level subroutines as well.
In the listings below, the
dly_us
subroutine causes the program thread to cease execution for the given
number of microseconds. The
SPI_TIMEOUT
constant is defined to provide something longer than a
character timeout.
In the high-level subroutines, an ENUM is used to select the register by name. It indexes the
register_lookup_table
array that, among other things, contains the register length for each of the
MAXQ3180 registers. See
Figures 6, 7,
and
8.
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