BCM20730 datasheet(13/62 Pages) CYPRESS

13 / 62 page

Functional Description

BROADCOM

September 9, 2013 • 20730-DS108-R

Page 12

BCM20730 Data Sheet

BROADCOM CONFIDENTIAL

Section 1: Functional Description

Keyboard Scanner

The keyboard scanner is designed to autonomously sample keys and store them into buffer registers without

the need for the host microcontroller to intervene. The scanner has the following features:

• Ability to turn off its clock if no keys pressed.

• Sequential scanning of up to 160 keys in an 8 x 20 matrix.

• Programmable number of columns from 1 to 20.

• Programmable number of rows from 1 to 8.

• 16-byte key-code buffer (can be augmented by firmware).

• 128 kHz clock – allows scanning of full 160-key matrix in about 1.2 ms.

• N-key rollover with selective 2-key lockout if ghost is detected.

• Keys are buffered until host microcontroller has a chance to read it, or until overflow occurs.

• Hardware debouncing and noise/glitch filtering.

• Low-power consumption. Single-digit µA-level sleep current.

Theory of Operation

The key scan block is controlled by a state machine with the following states:

Idle

The state machine begins in the idle state. In this state, all column outputs are driven high. If any key is pressed,

a transition occurs on one of the row inputs. This transition causes the 128 kHz clock to be enabled (if it is not

already enabled by another peripheral) and the state machine to enter the scan state. Also in this state, an 8-

bit row-hit register and an 8-bit key-index counter is reset to 0.

Scan

In the scan state, a row counter counts from 0 up to a programmable number of rows minus 1. Once the last

row is reached, the row counter is reset and the column counter is incremented. This cycle repeats until the

row and column counters are both at their respective terminal count values. At that point, the state machine

moves into the Scan-End state.

As the keys are being scanned, the key-index counter is incremented. This counter is the value compared to the

modifier key codes stored, or in the key-code buffer if the key is not a modifier key. It can be used by the

microprocessor as an index into a lookup table of usage codes.

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