Rainbow Electronics MAX5971B - Manual (Page 40)

40 _____________________________________________________________________________________
MAX5971B
Single-Port, 40W, IEEE 802.3af/at,
PSE Controller with I 2 C
Current Readout Registers
(R30h to R37h)
Port Current Registers (R30h to R31h)
The port current registers (R30h to R31h, Tables 38 and
39) provide port current readout during classification
and normal power mode. On a power-up or after a reset
condition, these registers are both set to a default value
of 0x00h. The port current readout has 9 bits of overall
resolution. The MAX5971B has 8-bit registers, so the
data is split between 2 consecutive registers. R30h[7:0]
contains the highest 8 bits (MSB) and R31h[0] contains
the lowest bit (LSB). To avoid the LSB register changing
while reading the MSB, the register contents are frozen if
the addressing byte points to either of the current read-
out registers.
When the port is powered, the port output current can
be calculated as:
I OUT = N IPD x 2.95mA
During classification, the port current is:
I
CLASS
= N
IPD
x 0.0975mA
where N IPD is the decimal value of the 9-bit port current
readout. The ADC saturates both at full scale and at
zero, resulting in poor current readout accuracy near the
top and bottom codes.
Reserved Registers (R32h to R37h)
Registers R32h to R37h are unconnected; writing to them
has no effect (address autoincrement still functions) and
a read always returns logical zeros.
Table 38. Port Current Register (MSB)
Table 39. Port Current Register (LSB)
ADDRESS = 30h
DESCRIPTION
SYMBOL BIT NO. R/W
IPD
7 R IPD[8] (MSB)
6 R IPD[7]
5 R IPD[6]
4 R IPD[5]
3 R IPD[4]
2 R IPD[3]
1 R IPD[2]
0 R IPD[1]
ADDRESS = 31h
DESCRIPTION
SYMBOL BIT NO. R/W
Reserved 7 — Reserved
Reserved 6 — Reserved
Reserved 5 — Reserved
Reserved 4 — Reserved
Reserved 3 — Reserved
Reserved 2 — Reserved
Reserved 1 — Reserved
IPD 0 R IPD[0] (LSB)