MSB

Serially Interfaced, 8-Digit LED Display Drivers

6

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_______________Detailed Description

MAX7219/MAX7221 Differences

The MAX7219 and MAX7221 are identical except for

two parameters: the MAX7221 segment drivers are

slew-rate limited to reduce electromagnetic interfer-

ence (EMI), and its serial interface is fully SPI compati-

ble.

Serial-Addressing Modes

For the MAX7219, serial data at DIN, sent in 16-bit

packets, is shifted into the internal 16-bit shift register

with each rising edge of CLK regardless of the state of

LOAD. For the MAX7221, CS must be low to clock data

in or out. The data is then latched into either the digit or

control registers on the rising edge of LOAD/CS.

LOAD/CS must go high concurrently with or after the

16th rising clock edge, but before the next rising clock

edge or data will be lost. Data at DIN is propagated

through the shift register and appears at DOUT 16.5

clock cycles later. Data is clocked out on the falling

edge of CLK. Data bits are labeled D0–D15 (Table 1).

D8–D11 contain the register address. D0–D7 contain

the data, and D12–D15 are “don’t care” bits. The first

received is D15, the most significant bit (MSB).

Digit and Control Registers

Table 2 lists the 14 addressable digit and control regis-

ters. The digit registers are realized with an on-chip,

8x8 dual-port SRAM. They are addressed directly so

that individual digits can be updated and retain data as

long as V+ typically exceeds 2V. The control registers

consist of decode mode, display intensity, scan limit

(number of scanned digits), shutdown, and display test

(all LEDs on).

Shutdown Mode

When the MAX7219 is in shutdown mode, the scan oscil-

lator is halted, all segment current sources are pulled to

ground, and all digit drivers are pulled to V+, thereby

blanking the display. The MAX7221 is identical, except

the drivers are high-impedance. Data in the digit and

control registers remains unaltered. Shutdown can be

used to save power or as an alarm to flash the display by

successively entering and leaving shutdown mode. For

minimum supply current in shutdown mode, logic inputs

should be at ground or V+ (CMOS-logic levels).

Typically, it takes less than 250µs for the MAX7219/

MAX7221 to leave shutdown mode. The display driver

can be programmed while in shutdown mode, and

shutdown mode can be overridden by the display-test

function.

CLK

DIN

D15

DOUT

D14

D1

D0

CS

OR LOAD

Figure 1. Timing Diagram

Table 1. Serial-Data Format (16 Bits)

D14

D15

X

D12

D13

X

X

D10

D11

D8

D9

ADDRESS

D6

D7

D4

D5

X

D2

D3

D0

D1

MSB

DATA

LSB