Accessing the MAX3420E Registers
An SPI™ master controls the MAX3420 by writing and reading twenty-one internal registers,
R0-R20. The SPI master begins every register access by asserting the MAX3420E SS# (slave
select, active low) pin, and clocking in eight bits that comprise the SPI command byte. Figure 1
shows the command byte format.
ACKSTAT
DIR
1=wr 0=rd
0
Reg0
Reg1
Reg2
Reg3
Reg4
b7
b6
b5
b4
b3
b2
b1
b0
Figure 1. SPI Command Byte. As for all SPI transfers, bit 7 is sent first.
Reg4:Reg0 set the register address, with valid values from 0-20. Values above 20 are ignored by
the MAX3420E. The direction bit sets the direction for the data transfer. The ACKSTAT bit
duplicates a USB control bit (R9 bit 6). ACKSTAT is provided in the control byte as a fast way
to set this often-used register bit.
After sending the command byte, the SPI master transfers one or more bytes in the direction
indicated by the DIR bit. Keeping SS# low, the SPI master provides additional bursts of eight
SCLK pulses for each byte. When the byte transfers are complete, the SPI master de-asserts SS#
(drives high) and the transfer terminates.
It is possible to truncate an SPI cycle after sending only the command byte. This feature allows
the SPI master to set the ACKSTAT bit without doing a full SPI access.
Note:
The ACKSTAT bit tells the MAX3420E that the SPI master has finished servicing a USB
Control transfer. This causes the MAX3420E to ACKnowledge the next Control transfer
STATUS stage.
To set the ACKSTAT bit using the SPI command byte, the SPI master sets the register field to a
dummy value (it won’t be used), sets the DIR bit for a read operation (for example, read
Revision register R18), and sets the ACKSTAT bit. The SPI master then asserts SS#, clocks in
the 8 command bits, and then truncates the SPI cycle by de-asserting the SS# signal. This is the
fastest way to set the ACKSTAT bit.
The MAX3420E has two register types, FIFOS and control registers. Repeated reads or writes to
a register has different effects, depending on the register type.
SPI is a trademark of Motorola, Inc.